def build_model(self):
#paper download url:https://arxiv.org/abs/1508.04025
#fig1
enc_in = Input(shape=(self.input_length, ),
dtype='int32',
name='enc_input')
enc_embedding = Embedding(input_dim=self.num_vocab,
output_dim=self.embedding_dim,
input_length=self.input_length,
trainable=True,
name='enc_embedding')
enc_embedded = enc_embedding(enc_in)
encoded, state = GRU(units=self.num_units,
return_sequences=True,
return_state=True,
name='enc_GRU')(enc_embedded)
#\fig1
################################
###### decoder model ############
##################################
#fig2
dec_in = Input(shape=(self.output_length, ),
dtype='int32',
name='dec_input')
dec_embedding = Embedding(input_dim=self.num_vocab,
output_dim=self.embedding_dim,
input_length=self.output_length,
trainable=True,
name='dec_embedding')
#share weights with encoder embedding layer
dec_embedding.embeddings = enc_embedding.embeddings
dec_embedded = dec_embedding(dec_in)
decoded = GRU(units=self.num_units,
return_sequences=True,
name='dec_GRU')(dec_embedded, initial_state=state)
#Luong's global attention
repeat_dec = TimeDistributed(RepeatVector(self.input_length),
name='repeat_dec')
rep_decoded = repeat_dec(decoded)
#/fig2
#fig3
annotation_layer = TimeDistributed(Dense(units=self.num_units),
name='annotation_layer')
annotation = annotation_layer(encoded)
repeat_enc = TimeDistributed(RepeatVector(self.output_length),
name='repeat_enc')
rep_annotation = repeat_enc(annotation)
rep_annotation = Permute(
(2, 1, 3),
input_shape=(self.input_length, self.output_length,
self.num_units),
name='permute_rep_annotation')(rep_annotation)
#fig4
attention_mul = Multiply(name='attention_mul')
elem_score = attention_mul([rep_decoded, rep_annotation])
score = Lambda(lambda x: K.sum(x, axis=3, keepdims=True),
name='score')(elem_score)
attention_weight = Lambda(lambda x: softmax(x, axis=2),
name='attention_weight')(score)
context_mul = Multiply(name='context_mul')
#\fig4
#fig5
rep_encoded = repeat_enc(encoded)
rep_encoded = Permute((2, 1, 3),
input_shape=(self.input_length,
self.output_length, self.num_units),
name='permute_rep_encoded')(rep_encoded)
elem_context = context_mul([rep_encoded, attention_weight])
context = Lambda(lambda x: K.sum(x, axis=2),
name='context')(elem_context)
concat = Concatenate(axis=-1)
dec_and_att = Lambda(lambda x: K.concatenate([x[0], x[1]], axis=-1),
name='dec_att_concat')([decoded, context])
#\fig5
#full_connection and output
#fig6
fc1 = TimeDistributed(Dense(units=self.num_units * 2),
name='fc1')(dec_and_att)
fc1_activated = Activation('tanh')(fc1)
fc2 = TimeDistributed(Dense(units=self.num_vocab),
name='fc2')(fc1_activated)
preds = Activation('softmax', name='softmax')(fc2)
#\fig6
model = Model([enc_in, dec_in], preds)
model.summary()
plot_model(model,
to_file='seq2seq_attention_model_plot.png',
show_shapes=True,
show_layer_names=True)
return model
