keras-monotonic-attention

seq2seq attention in keras

AttentionDecoder class is modified version of the one here https://github.com/datalogue/keras-attention

The main differences:

• internal embedding for output layers
• Luong-style monotonic attention (optional)
• attention weight regularization (optional)
• teacher forcing
• fixing minor bugs like https://github.com/datalogue/keras-attention/issues/30

Simple example

model = Sequential()
model.add(Embedding(INPUT_VOCABE_SIZE, INPUT_EMBEDDING_DIM))
model.add(Bidirectional(LSTM(150, return_sequences=True)))
model.add(AttentionDecoder(150, OUTPUT_VOCABE_SIZE, embedding_dim=OUTPUT_EMBEDDING_DIM))
model.compile(loss='sparse_categorical_crossentropy', optimizer='adam', metrics=['acc'])

See sequential_example.py for details

Attention mechanisms

To use Bahdanau [1] attention mechanism set is_monotonic=False and normalize_energy=False. Actually it is also default mode.

bahdanau_attention_decoder = AttentionDecoder(units, output_vocabe_size,
                                              is_monotonic=False,
                                              normalize_energy=False)

For Luong-style [2] attention set is_monotonic=True and normalize_energy=True. Note that it is still additive, not multiplicative.

luong_attention_decoder = AttentionDecoder(units, output_vocabe_size,
                                           is_monotonic=True,
                                           normalize_energy=True)

Teacher forcing

To enable teacher forcing use that way:

inputs = Input(shape=(None,), dtype='int64')
# encoder part
x = encoder_model(inputs)
# input that represent correct output sequence
labels_true = Input(shape=(None,), dtype='int64')

decoder_layer = AttentionDecoder(hidden_units, output_vocabe_size)

output = decoder_layer([x, labels_true])

See example.py for more details

Position embedding

One additional feature in this repo is PositionEmbedding. It is embedding for positional information. Just sin/cos ways with exponentionally. See [3], section 3.5, Position Encoding.

The layer should be used to extract some position information

References

[1] Bahdanau, Dzmitry, Kyunghyun Cho, and Yoshua Bengio. "Neural machine translation by jointly learning to align and translate." arXiv preprint arXiv:1409.0473 (2014).

[2] Colin Raffel, Minh-Thang Luong, Peter J. Liu, Ron J. Weiss, Douglass Eck "Online and Linear-Time Attention by Enforcing Monotonic Alignments" arXiv arXiv:1704.00784 (2017)

[3] Ashish Vaswani, Noam Shazeer, Niki Parmar, Jakob Uszkoreit, Llion Jones, Aidan N. Gomez, Lukasz Kaiser, Illia Polosukhin "Attention is all you need" arXiv arXiv:1706.03762