Joining the Transformer Encoder and Decoder Plus Masking

By Stefania Cristina on October 10, 2022 in Attention

Last Updated on January 6, 2023

We have arrived at a point where we have implemented and tested the Transformer encoder and decoder separately, and we may now join the two together into a complete model. We will also see how to create padding and look-ahead masks by which we will suppress the input values that will not be considered in the encoder or decoder computations. Our end goal remains to apply the complete model to Natural Language Processing (NLP).

In this tutorial, you will discover how to implement the complete Transformer model and create padding and look-ahead masks.

After completing this tutorial, you will know:

How to create a padding mask for the encoder and decoder
How to create a look-ahead mask for the decoder
How to join the Transformer encoder and decoder into a single model
How to print out a summary of the encoder and decoder layers

Let’s get started.

Joining the Transformer encoder and decoder and Masking
Photo by John O’Nolan, some rights reserved.

Tutorial Overview

This tutorial is divided into four parts; they are:

Recap of the Transformer Architecture
Masking
- Creating a Padding Mask
- Creating a Look-Ahead Mask
Joining the Transformer Encoder and Decoder
Creating an Instance of the Transformer Model
- Printing Out a Summary of the Encoder and Decoder Layers

Prerequisites

For this tutorial, we assume that you are already familiar with:

Recap of the Transformer Architecture

Recall having seen that the Transformer architecture follows an encoder-decoder structure. The encoder, on the left-hand side, is tasked with mapping an input sequence to a sequence of continuous representations; the decoder, on the right-hand side, receives the output of the encoder together with the decoder output at the previous time step to generate an output sequence.

The encoder-decoder structure of the Transformer architecture
Taken from “Attention Is All You Need“

In generating an output sequence, the Transformer does not rely on recurrence and convolutions.

You have seen how to implement the Transformer encoder and decoder separately. In this tutorial, you will join the two into a complete Transformer model and apply padding and look-ahead masking to the input values.

Let’s start first by discovering how to apply masking.

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Masking

Creating a Padding Mask

You should already be familiar with the importance of masking the input values before feeding them into the encoder and decoder.

As you will see when you proceed to train the Transformer model, the input sequences fed into the encoder and decoder will first be zero-padded up to a specific sequence length. The importance of having a padding mask is to make sure that these zero values are not processed along with the actual input values by both the encoder and decoder.

Let’s create the following function to generate a padding mask for both the encoder and decoder:

from tensorflow import math, cast, float32

def padding_mask(input):
    # Create mask which marks the zero padding values in the input by a 1
    mask = math.equal(input, 0)
    mask = cast(mask, float32)

    return mask

from tensorflow import math, cast, float32

def padding_mask(input):

# Create mask which marks the zero padding values in the input by a 1

mask = math.equal(input, 0)

mask = cast(mask, float32)

return mask

Upon receiving an input, this function will generate a tensor that marks by a value of one wherever the input contains a value of zero.

Hence, if you input the following array:

from numpy import array

input = array([1, 2, 3, 4, 0, 0, 0])
print(padding_mask(input))

from numpy import array

input = array([1, 2, 3, 4, 0, 0, 0])

print(padding_mask(input))

Then the output of the padding_mask function would be the following:

tf.Tensor([0. 0. 0. 0. 1. 1. 1.], shape=(7,), dtype=float32)

1	tf.Tensor([0. 0. 0. 0. 1. 1. 1.], shape=(7,), dtype=float32)

Creating a Look-Ahead Mask

A look-ahead mask is required to prevent the decoder from attending to succeeding words, such that the prediction for a particular word can only depend on known outputs for the words that come before it.

For this purpose, let’s create the following function to generate a look-ahead mask for the decoder:

from tensorflow import linalg, ones

def lookahead_mask(shape):
    # Mask out future entries by marking them with a 1.0
    mask = 1 - linalg.band_part(ones((shape, shape)), -1, 0)

    return mask

from tensorflow import linalg, ones

def lookahead_mask(shape):

# Mask out future entries by marking them with a 1.0

mask = 1 - linalg.band_part(ones((shape, shape)), -1, 0)

return mask

You will pass to it the length of the decoder input. Let’s make this length equal to 5, as an example:

print(lookahead_mask(5))

1	print(lookahead_mask(5))

Then the output that the lookahead_mask function returns is the following:

tf.Tensor(
[[0. 1. 1. 1. 1.]
 [0. 0. 1. 1. 1.]
 [0. 0. 0. 1. 1.]
 [0. 0. 0. 0. 1.]
 [0. 0. 0. 0. 0.]], shape=(5, 5), dtype=float32)

tf.Tensor(

[[0. 1. 1. 1. 1.]

[0. 0. 1. 1. 1.]

[0. 0. 0. 1. 1.]

[0. 0. 0. 0. 1.]

[0. 0. 0. 0. 0.]], shape=(5, 5), dtype=float32)

Again, the one values mask out the entries that should not be used. In this manner, the prediction of every word only depends on those that come before it.

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Joining the Transformer Encoder and Decoder

Let’s start by creating the class, TransformerModel, which inherits from the Model base class in Keras:

class TransformerModel(Model):
    def __init__(self, enc_vocab_size, dec_vocab_size, enc_seq_length, dec_seq_length, h, d_k, d_v, d_model, d_ff_inner, n, rate, **kwargs):
        super(TransformerModel, self).__init__(**kwargs)

        # Set up the encoder
        self.encoder = Encoder(enc_vocab_size, enc_seq_length, h, d_k, d_v, d_model, d_ff_inner, n, rate)

        # Set up the decoder
        self.decoder = Decoder(dec_vocab_size, dec_seq_length, h, d_k, d_v, d_model, d_ff_inner, n, rate)

        # Define the final dense layer
        self.model_last_layer = Dense(dec_vocab_size)
        ...

class TransformerModel(Model):

def __init__(self, enc_vocab_size, dec_vocab_size, enc_seq_length, dec_seq_length, h, d_k, d_v, d_model, d_ff_inner, n, rate, **kwargs):

super(TransformerModel, self).__init__(**kwargs)

# Set up the encoder

self.encoder = Encoder(enc_vocab_size, enc_seq_length, h, d_k, d_v, d_model, d_ff_inner, n, rate)

# Set up the decoder

self.decoder = Decoder(dec_vocab_size, dec_seq_length, h, d_k, d_v, d_model, d_ff_inner, n, rate)

# Define the final dense layer

self.model_last_layer = Dense(dec_vocab_size)

...

Our first step in creating the TransformerModel class is to initialize instances of the Encoder and Decoder classes implemented earlier and assign their outputs to the variables, encoder and decoder, respectively. If you saved these classes in separate Python scripts, do not forget to import them. I saved my code in the Python scripts encoder.py and decoder.py, so I need to import them accordingly.

You will also include one final dense layer that produces the final output, as in the Transformer architecture of Vaswani et al. (2017).

Next, you shall create the class method, call(), to feed the relevant inputs into the encoder and decoder.

A padding mask is first generated to mask the encoder input, as well as the encoder output, when this is fed into the second self-attention block of the decoder:

...
def call(self, encoder_input, decoder_input, training):

    # Create padding mask to mask the encoder inputs and the encoder outputs in the decoder
    enc_padding_mask = self.padding_mask(encoder_input)
...

...

def call(self, encoder_input, decoder_input, training):

# Create padding mask to mask the encoder inputs and the encoder outputs in the decoder

enc_padding_mask = self.padding_mask(encoder_input)

...

A padding mask and a look-ahead mask are then generated to mask the decoder input. These are combined together through an element-wise maximum operation:

...
# Create and combine padding and look-ahead masks to be fed into the decoder
dec_in_padding_mask = self.padding_mask(decoder_input)
dec_in_lookahead_mask = self.lookahead_mask(decoder_input.shape[1])
dec_in_lookahead_mask = maximum(dec_in_padding_mask, dec_in_lookahead_mask)
...

...

# Create and combine padding and look-ahead masks to be fed into the decoder

dec_in_padding_mask = self.padding_mask(decoder_input)

dec_in_lookahead_mask = self.lookahead_mask(decoder_input.shape[1])

dec_in_lookahead_mask = maximum(dec_in_padding_mask, dec_in_lookahead_mask)

...

Next, the relevant inputs are fed into the encoder and decoder, and the Transformer model output is generated by feeding the decoder output into one final dense layer:

...
# Feed the input into the encoder
encoder_output = self.encoder(encoder_input, enc_padding_mask, training)

# Feed the encoder output into the decoder
decoder_output = self.decoder(decoder_input, encoder_output, dec_in_lookahead_mask, enc_padding_mask, training)

# Pass the decoder output through a final dense layer
model_output = self.model_last_layer(decoder_output)

return model_output

...

# Feed the input into the encoder

encoder_output = self.encoder(encoder_input, enc_padding_mask, training)

# Feed the encoder output into the decoder

decoder_output = self.decoder(decoder_input, encoder_output, dec_in_lookahead_mask, enc_padding_mask, training)

# Pass the decoder output through a final dense layer

model_output = self.model_last_layer(decoder_output)

return model_output

Combining all the steps gives us the following complete code listing:

from encoder import Encoder
from decoder import Decoder
from tensorflow import math, cast, float32, linalg, ones, maximum, newaxis
from tensorflow.keras import Model
from tensorflow.keras.layers import Dense


class TransformerModel(Model):
    def __init__(self, enc_vocab_size, dec_vocab_size, enc_seq_length, dec_seq_length, h, d_k, d_v, d_model, d_ff_inner, n, rate, **kwargs):
        super(TransformerModel, self).__init__(**kwargs)

        # Set up the encoder
        self.encoder = Encoder(enc_vocab_size, enc_seq_length, h, d_k, d_v, d_model, d_ff_inner, n, rate)

        # Set up the decoder
        self.decoder = Decoder(dec_vocab_size, dec_seq_length, h, d_k, d_v, d_model, d_ff_inner, n, rate)

        # Define the final dense layer
        self.model_last_layer = Dense(dec_vocab_size)

    def padding_mask(self, input):
        # Create mask which marks the zero padding values in the input by a 1.0
        mask = math.equal(input, 0)
        mask = cast(mask, float32)

        # The shape of the mask should be broadcastable to the shape
        # of the attention weights that it will be masking later on
        return mask[:, newaxis, newaxis, :]

    def lookahead_mask(self, shape):
        # Mask out future entries by marking them with a 1.0
        mask = 1 - linalg.band_part(ones((shape, shape)), -1, 0)

        return mask

    def call(self, encoder_input, decoder_input, training):

        # Create padding mask to mask the encoder inputs and the encoder outputs in the decoder
        enc_padding_mask = self.padding_mask(encoder_input)

        # Create and combine padding and look-ahead masks to be fed into the decoder
        dec_in_padding_mask = self.padding_mask(decoder_input)
        dec_in_lookahead_mask = self.lookahead_mask(decoder_input.shape[1])
        dec_in_lookahead_mask = maximum(dec_in_padding_mask, dec_in_lookahead_mask)

        # Feed the input into the encoder
        encoder_output = self.encoder(encoder_input, enc_padding_mask, training)

        # Feed the encoder output into the decoder
        decoder_output = self.decoder(decoder_input, encoder_output, dec_in_lookahead_mask, enc_padding_mask, training)

        # Pass the decoder output through a final dense layer
        model_output = self.model_last_layer(decoder_output)

        return model_output

from encoder import Encoder

from decoder import Decoder

from tensorflow import math, cast, float32, linalg, ones, maximum, newaxis

from tensorflow.keras import Model

from tensorflow.keras.layers import Dense

class TransformerModel(Model):

def __init__(self, enc_vocab_size, dec_vocab_size, enc_seq_length, dec_seq_length, h, d_k, d_v, d_model, d_ff_inner, n, rate, **kwargs):

super(TransformerModel, self).__init__(**kwargs)

# Set up the encoder

self.encoder = Encoder(enc_vocab_size, enc_seq_length, h, d_k, d_v, d_model, d_ff_inner, n, rate)

# Set up the decoder

self.decoder = Decoder(dec_vocab_size, dec_seq_length, h, d_k, d_v, d_model, d_ff_inner, n, rate)

# Define the final dense layer

self.model_last_layer = Dense(dec_vocab_size)

def padding_mask(self, input):

# Create mask which marks the zero padding values in the input by a 1.0

mask = math.equal(input, 0)

mask = cast(mask, float32)

# The shape of the mask should be broadcastable to the shape

# of the attention weights that it will be masking later on

return mask[:, newaxis, newaxis, :]

def lookahead_mask(self, shape):

# Mask out future entries by marking them with a 1.0

mask = 1 - linalg.band_part(ones((shape, shape)), -1, 0)

return mask

def call(self, encoder_input, decoder_input, training):

# Create padding mask to mask the encoder inputs and the encoder outputs in the decoder

enc_padding_mask = self.padding_mask(encoder_input)

# Create and combine padding and look-ahead masks to be fed into the decoder

dec_in_padding_mask = self.padding_mask(decoder_input)

dec_in_lookahead_mask = self.lookahead_mask(decoder_input.shape[1])

dec_in_lookahead_mask = maximum(dec_in_padding_mask, dec_in_lookahead_mask)

# Feed the input into the encoder

encoder_output = self.encoder(encoder_input, enc_padding_mask, training)

# Feed the encoder output into the decoder

decoder_output = self.decoder(decoder_input, encoder_output, dec_in_lookahead_mask, enc_padding_mask, training)

# Pass the decoder output through a final dense layer

model_output = self.model_last_layer(decoder_output)

return model_output

Note that you have performed a small change to the output that is returned by the padding_mask function. Its shape is made broadcastable to the shape of the attention weight tensor that it will mask when you train the Transformer model.

Creating an Instance of the Transformer Model

You will work with the parameter values specified in the paper, Attention Is All You Need, by Vaswani et al. (2017):

h = 8  # Number of self-attention heads
d_k = 64  # Dimensionality of the linearly projected queries and keys
d_v = 64  # Dimensionality of the linearly projected values
d_ff = 2048  # Dimensionality of the inner fully connected layer
d_model = 512  # Dimensionality of the model sub-layers' outputs
n = 6  # Number of layers in the encoder stack

dropout_rate = 0.1  # Frequency of dropping the input units in the dropout layers
...

h = 8 # Number of self-attention heads

d_k = 64 # Dimensionality of the linearly projected queries and keys

d_v = 64 # Dimensionality of the linearly projected values

d_ff = 2048 # Dimensionality of the inner fully connected layer

d_model = 512 # Dimensionality of the model sub-layers' outputs

n = 6 # Number of layers in the encoder stack

dropout_rate = 0.1 # Frequency of dropping the input units in the dropout layers

...

As for the input-related parameters, you will work with dummy values for now until you arrive at the stage of training the complete Transformer model. At that point, you will use actual sentences:

...
enc_vocab_size = 20 # Vocabulary size for the encoder
dec_vocab_size = 20 # Vocabulary size for the decoder

enc_seq_length = 5  # Maximum length of the input sequence
dec_seq_length = 5  # Maximum length of the target sequence
...

...

enc_vocab_size = 20 # Vocabulary size for the encoder

dec_vocab_size = 20 # Vocabulary size for the decoder

enc_seq_length = 5 # Maximum length of the input sequence

dec_seq_length = 5 # Maximum length of the target sequence

...

You can now create an instance of the TransformerModel class as follows:

from model import TransformerModel

# Create model
training_model = TransformerModel(enc_vocab_size, dec_vocab_size, enc_seq_length, dec_seq_length, h, d_k, d_v, d_model, d_ff, n, dropout_rate)

from model import TransformerModel

# Create model

training_model = TransformerModel(enc_vocab_size, dec_vocab_size, enc_seq_length, dec_seq_length, h, d_k, d_v, d_model, d_ff, n, dropout_rate)

The complete code listing is as follows:

enc_vocab_size = 20 # Vocabulary size for the encoder
dec_vocab_size = 20 # Vocabulary size for the decoder

enc_seq_length = 5  # Maximum length of the input sequence
dec_seq_length = 5  # Maximum length of the target sequence

h = 8  # Number of self-attention heads
d_k = 64  # Dimensionality of the linearly projected queries and keys
d_v = 64  # Dimensionality of the linearly projected values
d_ff = 2048  # Dimensionality of the inner fully connected layer
d_model = 512  # Dimensionality of the model sub-layers' outputs
n = 6  # Number of layers in the encoder stack

dropout_rate = 0.1  # Frequency of dropping the input units in the dropout layers

# Create model
training_model = TransformerModel(enc_vocab_size, dec_vocab_size, enc_seq_length, dec_seq_length, h, d_k, d_v, d_model, d_ff, n, dropout_rate)

enc_vocab_size = 20 # Vocabulary size for the encoder

dec_vocab_size = 20 # Vocabulary size for the decoder

enc_seq_length = 5 # Maximum length of the input sequence

dec_seq_length = 5 # Maximum length of the target sequence

h = 8 # Number of self-attention heads

d_k = 64 # Dimensionality of the linearly projected queries and keys

d_v = 64 # Dimensionality of the linearly projected values

d_ff = 2048 # Dimensionality of the inner fully connected layer

d_model = 512 # Dimensionality of the model sub-layers' outputs

n = 6 # Number of layers in the encoder stack

dropout_rate = 0.1 # Frequency of dropping the input units in the dropout layers

# Create model

training_model = TransformerModel(enc_vocab_size, dec_vocab_size, enc_seq_length, dec_seq_length, h, d_k, d_v, d_model, d_ff, n, dropout_rate)

Printing Out a Summary of the Encoder and Decoder Layers

You may also print out a summary of the encoder and decoder blocks of the Transformer model. The choice to print them out separately will allow you to be able to see the details of their individual sub-layers. In order to do so, add the following line of code to the __init__() method of both the EncoderLayer and DecoderLayer classes:

self.build(input_shape=[None, sequence_length, d_model])

1	self.build(input_shape=[None, sequence_length, d_model])

Then you need to add the following method to the EncoderLayer class:

def build_graph(self):
    input_layer = Input(shape=(self.sequence_length, self.d_model))
    return Model(inputs=[input_layer], outputs=self.call(input_layer, None, True))

def build_graph(self):

input_layer = Input(shape=(self.sequence_length, self.d_model))

return Model(inputs=[input_layer], outputs=self.call(input_layer, None, True))

And the following method to the DecoderLayer class:

def build_graph(self):
    input_layer = Input(shape=(self.sequence_length, self.d_model))
    return Model(inputs=[input_layer], outputs=self.call(input_layer, input_layer, None, None, True))

def build_graph(self):

input_layer = Input(shape=(self.sequence_length, self.d_model))

return Model(inputs=[input_layer], outputs=self.call(input_layer, input_layer, None, None, True))

This results in the EncoderLayer class being modified as follows (the three dots under the call() method mean that this remains the same as the one that was implemented here):

from tensorflow.keras.layers import Input
from tensorflow.keras import Model

class EncoderLayer(Layer):
    def __init__(self, sequence_length, h, d_k, d_v, d_model, d_ff, rate, **kwargs):
        super(EncoderLayer, self).__init__(**kwargs)
        self.build(input_shape=[None, sequence_length, d_model])
        self.d_model = d_model
        self.sequence_length = sequence_length
        self.multihead_attention = MultiHeadAttention(h, d_k, d_v, d_model)
        self.dropout1 = Dropout(rate)
        self.add_norm1 = AddNormalization()
        self.feed_forward = FeedForward(d_ff, d_model)
        self.dropout2 = Dropout(rate)
        self.add_norm2 = AddNormalization()

    def build_graph(self):
        input_layer = Input(shape=(self.sequence_length, self.d_model))
        return Model(inputs=[input_layer], outputs=self.call(input_layer, None, True))

    def call(self, x, padding_mask, training):
        ...

from tensorflow.keras.layers import Input

from tensorflow.keras import Model

class EncoderLayer(Layer):

def __init__(self, sequence_length, h, d_k, d_v, d_model, d_ff, rate, **kwargs):

super(EncoderLayer, self).__init__(**kwargs)

self.build(input_shape=[None, sequence_length, d_model])

self.d_model = d_model

self.sequence_length = sequence_length

self.multihead_attention = MultiHeadAttention(h, d_k, d_v, d_model)

self.dropout1 = Dropout(rate)

self.add_norm1 = AddNormalization()

self.feed_forward = FeedForward(d_ff, d_model)

self.dropout2 = Dropout(rate)

self.add_norm2 = AddNormalization()

def build_graph(self):

input_layer = Input(shape=(self.sequence_length, self.d_model))

return Model(inputs=[input_layer], outputs=self.call(input_layer, None, True))

def call(self, x, padding_mask, training):

...

Similar changes can be made to the DecoderLayer class too.

Once you have the necessary changes in place, you can proceed to create instances of the EncoderLayer and DecoderLayer classes and print out their summaries as follows:

from encoder import EncoderLayer
from decoder import DecoderLayer

encoder = EncoderLayer(enc_seq_length, h, d_k, d_v, d_model, d_ff, dropout_rate)
encoder.build_graph().summary()

decoder = DecoderLayer(dec_seq_length, h, d_k, d_v, d_model, d_ff, dropout_rate)
decoder.build_graph().summary()

from encoder import EncoderLayer

from decoder import DecoderLayer

encoder = EncoderLayer(enc_seq_length, h, d_k, d_v, d_model, d_ff, dropout_rate)

encoder.build_graph().summary()

decoder = DecoderLayer(dec_seq_length, h, d_k, d_v, d_model, d_ff, dropout_rate)

decoder.build_graph().summary()

The resulting summary for the encoder is the following:

Model: "model"
__________________________________________________________________________________________________
 Layer (type)                   Output Shape         Param #     Connected to                     
==================================================================================================
 input_1 (InputLayer)           [(None, 5, 512)]     0           []                               
                                                                                                  
 multi_head_attention_18 (Multi  (None, 5, 512)      131776      ['input_1[0][0]',                
 HeadAttention)                                                   'input_1[0][0]',                
                                                                  'input_1[0][0]']                
                                                                                                  
 dropout_32 (Dropout)           (None, 5, 512)       0           ['multi_head_attention_18[0][0]']
                                                                                                  
 add_normalization_30 (AddNorma  (None, 5, 512)      1024        ['input_1[0][0]',                
 lization)                                                        'dropout_32[0][0]']             
                                                                                                  
 feed_forward_12 (FeedForward)  (None, 5, 512)       2099712     ['add_normalization_30[0][0]']   
                                                                                                  
 dropout_33 (Dropout)           (None, 5, 512)       0           ['feed_forward_12[0][0]']        
                                                                                                  
 add_normalization_31 (AddNorma  (None, 5, 512)      1024        ['add_normalization_30[0][0]',   
 lization)                                                        'dropout_33[0][0]']             
                                                                                                  
==================================================================================================
Total params: 2,233,536
Trainable params: 2,233,536
Non-trainable params: 0
__________________________________________________________________________________________________

Model: "model"

__________________________________________________________________________________________________

Layer (type) Output Shape Param # Connected to

==================================================================================================

input_1 (InputLayer) [(None, 5, 512)] 0 []

multi_head_attention_18 (Multi (None, 5, 512) 131776 ['input_1[0][0]',

HeadAttention) 'input_1[0][0]',

'input_1[0][0]']

dropout_32 (Dropout) (None, 5, 512) 0 ['multi_head_attention_18[0][0]']

add_normalization_30 (AddNorma (None, 5, 512) 1024 ['input_1[0][0]',

lization) 'dropout_32[0][0]']

feed_forward_12 (FeedForward) (None, 5, 512) 2099712 ['add_normalization_30[0][0]']

dropout_33 (Dropout) (None, 5, 512) 0 ['feed_forward_12[0][0]']

add_normalization_31 (AddNorma (None, 5, 512) 1024 ['add_normalization_30[0][0]',

lization) 'dropout_33[0][0]']

==================================================================================================

Total params: 2,233,536

Trainable params: 2,233,536

Non-trainable params: 0

__________________________________________________________________________________________________

While the resulting summary for the decoder is the following:

Model: "model_1"
__________________________________________________________________________________________________
 Layer (type)                   Output Shape         Param #     Connected to                     
==================================================================================================
 input_2 (InputLayer)           [(None, 5, 512)]     0           []                               
                                                                                                  
 multi_head_attention_19 (Multi  (None, 5, 512)      131776      ['input_2[0][0]',                
 HeadAttention)                                                   'input_2[0][0]',                
                                                                  'input_2[0][0]']                
                                                                                                  
 dropout_34 (Dropout)           (None, 5, 512)       0           ['multi_head_attention_19[0][0]']
                                                                                                  
 add_normalization_32 (AddNorma  (None, 5, 512)      1024        ['input_2[0][0]',                
 lization)                                                        'dropout_34[0][0]',             
                                                                  'add_normalization_32[0][0]',   
                                                                  'dropout_35[0][0]']             
                                                                                                  
 multi_head_attention_20 (Multi  (None, 5, 512)      131776      ['add_normalization_32[0][0]',   
 HeadAttention)                                                   'input_2[0][0]',                
                                                                  'input_2[0][0]']                
                                                                                                  
 dropout_35 (Dropout)           (None, 5, 512)       0           ['multi_head_attention_20[0][0]']
                                                                                                  
 feed_forward_13 (FeedForward)  (None, 5, 512)       2099712     ['add_normalization_32[1][0]']   
                                                                                                  
 dropout_36 (Dropout)           (None, 5, 512)       0           ['feed_forward_13[0][0]']        
                                                                                                  
 add_normalization_34 (AddNorma  (None, 5, 512)      1024        ['add_normalization_32[1][0]',   
 lization)                                                        'dropout_36[0][0]']             
                                                                                                  
==================================================================================================
Total params: 2,365,312
Trainable params: 2,365,312
Non-trainable params: 0
__________________________________________________________________________________________________

Model: "model_1"

__________________________________________________________________________________________________

Layer (type) Output Shape Param # Connected to

==================================================================================================

input_2 (InputLayer) [(None, 5, 512)] 0 []

multi_head_attention_19 (Multi (None, 5, 512) 131776 ['input_2[0][0]',

HeadAttention) 'input_2[0][0]',

'input_2[0][0]']

dropout_34 (Dropout) (None, 5, 512) 0 ['multi_head_attention_19[0][0]']

add_normalization_32 (AddNorma (None, 5, 512) 1024 ['input_2[0][0]',

lization) 'dropout_34[0][0]',

'add_normalization_32[0][0]',

'dropout_35[0][0]']

multi_head_attention_20 (Multi (None, 5, 512) 131776 ['add_normalization_32[0][0]',

HeadAttention) 'input_2[0][0]',

'input_2[0][0]']

dropout_35 (Dropout) (None, 5, 512) 0 ['multi_head_attention_20[0][0]']

feed_forward_13 (FeedForward) (None, 5, 512) 2099712 ['add_normalization_32[1][0]']

dropout_36 (Dropout) (None, 5, 512) 0 ['feed_forward_13[0][0]']

add_normalization_34 (AddNorma (None, 5, 512) 1024 ['add_normalization_32[1][0]',

lization) 'dropout_36[0][0]']

==================================================================================================

Total params: 2,365,312

Trainable params: 2,365,312

Non-trainable params: 0

__________________________________________________________________________________________________

Summary

In this tutorial, you discovered how to implement the complete Transformer model and create padding and look-ahead masks.

Specifically, you learned:

How to create a padding mask for the encoder and decoder
How to create a look-ahead mask for the decoder
How to join the Transformer encoder and decoder into a single model
How to print out a summary of the encoder and decoder layers

Do you have any questions?
Ask your questions in the comments below and I will do my best to answer.

21 Responses to Joining the Transformer Encoder and Decoder Plus Masking

Helen October 16, 2022 at 3:44 am #

Thanks for your great tutorial!
I found some errors when printing out the encoder and decoder structure information.
It seems the reason is in the MultiHeadAttention/reshape_tensor function. Precisely, reshape is not correctly working for tensors with None dimension.
I solved the problem by changing the function as below.
_________________________________________________
def reshape_tensor(self, x, heads, flag):
if flag:
# Tensor shape after reshaping and transposing: (batch_size, heads, seq_length, -1)
x = reshape(x, shape=(shape(x)[0], shape(x)[1], heads, int(x.shape[2]/heads)))
x = transpose(x, perm=(0, 2, 1, 3))
else:
# Reverting the reshaping and transposing operations: (batch_size, seq_length, d_model)
x = transpose(x, perm=(0, 2, 1, 3))
x = reshape(x, shape=(shape(x)[0], shape(x)[1], int(x.shape[2]*x.shape[3])))
return x
_________________________________________________
It seems weird somehow. I want to know if there are any better solutions. Thanks again!

Reply
- James Carmichael October 17, 2022 at 10:45 am #
  
  Thank you Helen for your feedback! What specific error messages did you encounter so that we may take note of any code listings that require review.
  
  Reply
  - Helen October 17, 2022 at 11:00 pm #
    
    Thanks for your reply! The error is like below.
    
    in test_transformer
    encoder.build_graph().summary()
    in build_graph
    return Model(inputs=[input_layer], outputs=self.call(input_layer, None, True))
    in call
    multihead_output = self.multihead_attention(x, x, x, padding_mask)
    in call *
    return self.W_o(output)
    ValueError: The last dimension of the inputs to a Dense layer should be defined. Found None. Full input shape received: (None, 5, None)
    Call arguments received by layer “multi_head_attention_18” (type MultiHeadAttention):
    \u2022 queries=tf.Tensor(shape=(None, 5, 512), dtype=float32)
    \u2022 keys=tf.Tensor(shape=(None, 5, 512), dtype=float32)
    \u2022 values=tf.Tensor(shape=(None, 5, 512), dtype=float32)
    \u2022 mask=None
    
    Reply
Chunhua October 29, 2022 at 8:36 am #

The last part of adding graph output is quite confusing.
It invovles many changes in different places.
In the end, I still got some error:

49
50 def build_graph(self):
—> 51 input_layer = Input(shape=(self.sequence_length, self.d_model))
52 return Model(inputs=[input_layer], outputs=self.call(input_layer, None, True))
53 def call(self, x, padding_mask, training):

The collab reproducing the error:

NameError: name ‘Input’ is not defined

Reply
- michel November 19, 2022 at 8:34 pm #
  
  I had the same error.
  
  Reply
  - Ivan February 26, 2023 at 10:09 am #
    
    Input layer didn’t imported in import section. Append Input at the end of the first line
    
    from tensorflow.keras.layers import LayerNormalization, Layer, Dense, ReLU, Dropout, Input
    
    Reply
michel November 19, 2022 at 8:08 pm #

I got the following error when building the decoder summary, same as encoder summary

TypeError Traceback (most recent call last)
in
2 # encoder.build_graph().summary()
3
—-> 4 decoder = DecoderLayer(dec_seq_length, h, d_k, d_v, d_model, d_ff, dropout_rate)
5 decoder.build_graph().summary()

TypeError: __init__() takes 7 positional arguments but 8 were given

Reply
- James Carmichael November 20, 2022 at 11:54 am #
  
  Hi Michel…I would recommend trying the code in Google Colab.
  
  Reply
- Ivan February 26, 2023 at 10:15 am #
  
  Check the __init__ function in the DecoderLayer class.
  Book version before this chapter don’t have dec_seq_length (only 7 input parameter), but if you look closely at the constructor in this chapter there are multiple changes that are not mentioned in the text:
  
  class EncoderLayer(Layer):
  # ——— sequence_length added
  def __init__(self, sequence_length, h, d_k, d_v, d_model, d_ff, rate, **kwargs):
  super().__init__(**kwargs)
  self.build(input_shape=[None, sequence_length, d_model])
  # ——– below two lines are added
  self.d_model = d_model
  self.sequence_length = sequence_length
  self.multihead_attention = MultiHeadAttention(h, d_k, d_v, d_model)
  self.dropout1 = Dropout(rate)
  self.add_norm1 = AddNormalization()
  self.feed_forward = FeedForward(d_ff, d_model)
  self.dropout2 = Dropout(rate)
  
  If you introduce all these changes, then build_graph will work, but everything else most probably will be broken
  
  Reply
Michel November 19, 2022 at 8:36 pm #

ValueError Traceback (most recent call last)
in
2
3 encoder = EncoderLayer(enc_seq_length, h, d_k, d_v, d_model, d_ff, dropout_rate)
—-> 4 encoder.build_graph().summary()
5
6 decoder = DecoderLayer(dec_seq_length, h, d_k, d_v, d_model, d_ff, dropout_rate)

3 frames
/tmp/__autograph_generated_file2sd23ix_.py in tf__call(self, queries, keys, values, mask)
15 try:
16 do_return = True
—> 17 retval_ = ag__.converted_call(ag__.ld(self).W_o, (ag__.ld(output),), None, fscope)
18 except:
19 do_return = False

ValueError: Exception encountered when calling layer “multi_head_attention_155” (type MultiHeadAttention).

in user code:

File “”, line 48, in call *
return self.W_o(output)
File “/usr/local/lib/python3.7/dist-packages/keras/utils/traceback_utils.py”, line 67, in error_handler **
raise e.with_traceback(filtered_tb) from None
File “/usr/local/lib/python3.7/dist-packages/keras/layers/core/dense.py”, line 141, in build
raise ValueError(‘The last dimension of the inputs to a Dense layer ‘

ValueError: The last dimension of the inputs to a Dense layer should be defined. Found None. Full input shape received: (None, 5, None)

Call arguments received by layer “multi_head_attention_155” (type MultiHeadAttention):
• queries=tf.Tensor(shape=(None, 5, 512), dtype=float32)
• keys=tf.Tensor(shape=(None, 5, 512), dtype=float32)
• values=tf.Tensor(shape=(None, 5, 512), dtype=float32)
• mask=None

Reply
- James Carmichael November 20, 2022 at 11:51 am #
  
  Hi Michel…Did you type in the code or copy and paste it? You may wish to try it Google Colab.
  
  Reply
- Ivan February 26, 2023 at 10:18 am #
  
  This error appears due to constructor in the class EncoderLayer changed, but class Encoder still rely upon previous implementation. To fix the problem Encoder-class must take into account new constructor-parameter seq_length.
  
  Reply
Shivam Shrivastava February 11, 2023 at 4:58 am #

class Transformer(tf.keras.Model):
def __init__(self, num_layers, d_model, num_heads, dff, input_vocab_size, target_vocab_size, pe_input, pe_target, rate=0.1):
super(Transformer, self).__init__()

self.encoder = Encoder(num_layers, d_model, num_heads, dff, input_vocab_size, pe_input, rate)

self.decoder = Decoder(num_layers, d_model, num_heads, dff, target_vocab_size, pe_target, rate)

self.final_layer = tf.keras.layers.Dense(target_vocab_size)

def call(self, inp, tar, training, enc_padding_mask, look_ahead_mask, dec_padding_mask):
enc_output = self.encoder(inp, training, enc_padding_mask)

dec_output, attention_weights = self.decoder(tar, enc_output, training, look_ahead_mask, dec_padding_mask)

final_output = self.final_layer(dec_output)

return final_output, attention_weights

NOTE: rest of the code for transformer/training of transformer is similar to these blogs
.
.
.
.
.
.

Here is my Transformer class… during training this subclass model, after every 3 epochs I was trying to save the subclass model using this line of code: `trainer.save(‘net’, save_format=’tf’)

but I was getting this error:
TypeError: tf__call() missing 3 required positional arguments: ‘enc_padding_mask’, ‘look_ahead_mask’, and ‘dec_padding_mask’

could you please suggest me the solution of it ?

Reply
- James Carmichael February 11, 2023 at 7:41 am #
  
  Hi Shivam…The following discussion may prove beneficial:
  
  https://stackoverflow.com/questions/61631360/self-defined-tensorflow-decoder-typeerror-call-missing-1-required-positio
  
  Reply
- Ivan February 26, 2023 at 10:22 am #
  
  Hello,
  
  your call function declaration is
  def call(self, inp, tar, training, enc_padding_mask, look_ahead_mask, dec_padding_mask):
  
  which has three new mask-parameters.
  
  If rest code is the same as in the book, then Transformer.call() doesn’t have these new parameters.
  
  Reply
Vik February 23, 2023 at 5:51 pm #

Hi,
Thanks for a great tutorial! How can I implement this is in transformers package by Hugging face?

Reply
- James Carmichael February 24, 2023 at 11:04 am #
  
  Hi Vik…The following resource is a great starting point:
  
  https://machinelearningmastery.com/a-brief-introduction-to-bert/
  
  Reply
Ivan February 25, 2023 at 3:05 pm #

Hello experts,

While reading this chapter and running many experiments, I noticed a behavior that I’d think is incorrect. Wanted to know your opinion.

Note: in the below explanation
*) loosely following types int vs float
*) loosely using terms “tokens” vs “words”, which is the same in this case
Sorry for that.

— Padding mask —

let’s assume:
input_seq_len = 9 # words in encoded sentence
input_seq = [[2,3,4,5,0,0,0,0,0]]

In class TransformerModel.padding_mask:
mask = padding_mask(input_seq)
# tensor [[[[0,0,0,0,1,1,1,1,1]]]], shape (1,1,1,9)
# first “1” is a batch
# second and third “1”-s are tf.newaxis
# last dimension is an actual mask

Idea of this mask is to prohibit “0”-token to attend to actual words (2,3,4,5)

Let’s see what is happening in DotProductAttention.call line:
scores += -1e9 * mask
# scores.shape (1, heads, 9, 9)
# “1” is a batch
# 9×9 is an attention score matrix, which shows how each token must attend to another token

When tensorflow multiplies (9, 9)-score to (1, 9)-mask matrices broadcast rule applies. Which produces scores in the following form (* is -1e9):
[x11, x12, x13, x14, *,*,*,*,*]
[x21, x22, x23, x24, *,*,*,*,*]
[x31, x32, x33, x34, *,*,*,*,*]
[x41, x42, x43, x44, *,*,*,*,*]
[x51, x52, x53, x54, *,*,*,*,*]
[x61, x62, x63, x64, *,*,*,*,*]
[x71, x72, x73, x74, *,*,*,*,*]
[x81, x82, x83, x84, *,*,*,*,*]
[x91, x92, x93, x94, *,*,*,*,*]

Observtion 1)
Above matrix allows 5-th, 6-th, 7-th, 8-th and 9-th tokens attend to 1-st, 2-nd, 3-rd and 4-th. I think above matrix must be
[x11, x12, x13, x14, *,*,*,*,*]
[x21, x22, x23, x24, *,*,*,*,*]
[x31, x32, x33, x34, *,*,*,*,*]
[x41, x42, x43, x44, *,*,*,*,*]
[ *, *, *, *, *,*,*,*,*]
[ *, *, *, *, *,*,*,*,*]
[ *, *, *, *, *,*,*,*,*]
[ *, *, *, *, *,*,*,*,*]
[ *, *, *, *, *,*,*,*,*]

Solution 1)
If observation 1 is correct
def PaddingMask(self, input):
mask = tf.equal(input, 0)
mask = tf.cast(mask, tf.float32)
mask = mask[:, tf.newaxis, tf.newaxis, :]
new_mask = tf.maximum(mask, tf.transpose(mask, perm=(0,1,3,2)))
return new_mask

— Lookahead mask combined with padding mask —
Very same setup.

In the TransformerModel.call():
dec_lookahead_mask = tf.maximum(dec_lookahead_mask_pre, dec_padding_mask)
will looks like:

[1,0,0,0,0,0,0,0,0]
[1,1,0,0,0,0,0,0,0]
[1,1,1,0,0,0,0,0,0]
[1,1,1,1,0,0,0,0,0]
[1,1,1,1,0,0,0,0,0]
[1,1,1,1,0,0,0,0,0]
[1,1,1,1,0,0,0,0,0]
[1,1,1,1,0,0,0,0,0]
[1,1,1,1,0,0,0,0,0]

But should be:
[1,0,0,0,0,0,0,0,0]
[1,1,0,0,0,0,0,0,0]
[1,1,1,0,0,0,0,0,0]
[1,1,1,1,0,0,0,0,0]
[0,0,0,0,0,0,0,0,0]
[0,0,0,0,0,0,0,0,0]
[0,0,0,0,0,0,0,0,0]
[0,0,0,0,0,0,0,0,0]
[0,0,0,0,0,0,0,0,0]

Observation 2)
Above mask allows attend zero-tokens to actual words.

Solution 2)
If observation 2 is valid, Solution(1) will fix Observation (2) as well

Reply
- Jamol February 28, 2023 at 2:26 am #
  
  I totally agree with your observation. Zero tokens should not be allow to attend to actual tokens so score matrix should be blockwise non-zero. However, your solution generates smth like
  
  array([[[[0., 0., 1., 1.],
  [0., 0., 1., 1.],
  [1., 1., 1., 1.],
  [1., 1., 1., 1.]]]], dtype=float32)>
  
  if we have, for example, a sequence [2,4,0,0]. This array does not work in the line
  
  scores = scores + mask * -1e9
  
  in the implementation of masking in dot product attention. The reason is obviously due to softmax operating row-wise: attention scores of zero tokens will ofc change by -1e9, however, all of them uniformly do so and softmax eventually normalizes them to equal values.
  
  Reply
  - Ivan February 28, 2023 at 2:30 pm #
    
    In my message I messed up with ones and zeros, but code still correct.
    In a padding/lookahead mask 1 means – should be masked.
    When 1 multiplies to -1e9 it became very small number. So mask should have ones in the position that must be masked.
    
    Reply
Ivan February 28, 2023 at 2:20 pm #

scores += -1e9 * mask
# here are shapes:
# (1, 8, 4, 4) + -1e9 * (1, 1, 4, 4) – works ok, due to broadcasting

Reply

Navigation

Joining the Transformer Encoder and Decoder Plus Masking

Tutorial Overview

Prerequisites

Recap of the Transformer Architecture

Masking

Creating a Padding Mask

Creating a Look-Ahead Mask

Want to Get Started With Building Transformer Models with Attention?

Joining the Transformer Encoder and Decoder

Creating an Instance of the Transformer Model

Printing Out a Summary of the Encoder and Decoder Layers

Further Reading

Books

Papers

Summary

Learn Transformers and Attention!

Teach your deep learning model to read a sentence

Give magical power of understanding human language for
Your Projects

More On This Topic

21 Responses to Joining the Transformer Encoder and Decoder Plus Masking

Leave a Reply Click here to cancel reply.

Navigation

Tutorial Overview

Prerequisites

Recap of the Transformer Architecture

Masking

Creating a Padding Mask

Creating a Look-Ahead Mask

Want to Get Started With Building Transformer Models with Attention?

Joining the Transformer Encoder and Decoder

Creating an Instance of the Transformer Model

Printing Out a Summary of the Encoder and Decoder Layers

Further Reading

Books

Papers

Summary

Learn Transformers and Attention!

Teach your deep learning model to read a sentence

Give magical power of understanding human language for Your Projects

More On This Topic

21 Responses to Joining the Transformer Encoder and Decoder Plus Masking

Leave a Reply Click here to cancel reply.

Give magical power of understanding human language for
Your Projects