tf.contrib.seq2seq.BahdanauAttention

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Implements Bahdanau-style (additive) attention.

tf.contrib.seq2seq.BahdanauAttention(
    num_units, memory, memory_sequence_length=None, normalize=False,
    probability_fn=None, score_mask_value=None, dtype=None,
    custom_key_value_fn=None, name='BahdanauAttention'
)

This attention has two forms. The first is Bahdanau attention, as described in:

Dzmitry Bahdanau, Kyunghyun Cho, Yoshua Bengio. "Neural Machine Translation by Jointly Learning to Align and Translate." ICLR 2015. https://arxiv.org/abs/1409.0473

The second is the normalized form. This form is inspired by the weight normalization article:

Tim Salimans, Diederik P. Kingma. "Weight Normalization: A Simple Reparameterization to Accelerate Training of Deep Neural Networks." https://arxiv.org/abs/1602.07868

To enable the second form, construct the object with parameter normalize=True.

num_units The depth of the query mechanism.
memory The memory to query; usually the output of an RNN encoder. This tensor should be shaped [batch_size, max_time, ...].
memory_sequence_length (optional) Sequence lengths for the batch entries in memory. If provided, the memory tensor rows are masked with zeros for values past the respective sequence lengths.
normalize Python boolean. Whether to normalize the energy term.
probability_fn (optional) A callable. Converts the score to probabilities. The default is tf.nn.softmax. Other options include tf.contrib.seq2seq.hardmax and tf.contrib.sparsemax.sparsemax. Its signature should be: probabilities = probability_fn(score).
score_mask_value (optional): The mask value for score before passing into probability_fn. The default is -inf. Only used if memory_sequence_length is not None.
dtype The data type for the query and memory layers of the attention mechanism.
custom_key_value_fn (optional): The custom function for computing keys and values.
name Name to use when creating ops.

alignments_size

batch_size

keys

memory_layer

query_layer

state_size

values

Methods

initial_alignments

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initial_alignments(
    batch_size, dtype
)

Creates the initial alignment values for the AttentionWrapper class.

This is important for AttentionMechanisms that use the previous alignment to calculate the alignment at the next time step (e.g. monotonic attention).

The default behavior is to return a tensor of all zeros.

Args
batch_size int32 scalar, the batch_size.
dtype The dtype.

Returns
A dtype tensor shaped [batch_size, alignments_size] (alignments_size is the values' max_time).

initial_state

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initial_state(
    batch_size, dtype
)

Creates the initial state values for the AttentionWrapper class.

This is important for AttentionMechanisms that use the previous alignment to calculate the alignment at the next time step (e.g. monotonic attention).

The default behavior is to return the same output as initial_alignments.

Args
batch_size int32 scalar, the batch_size.
dtype The dtype.

Returns
A structure of all-zero tensors with shapes as described by state_size.

__call__

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__call__(
    query, state
)

Score the query based on the keys and values.

Args
query Tensor of dtype matching self.values and shape [batch_size, query_depth].
state Tensor of dtype matching self.values and shape [batch_size, alignments_size] (alignments_size is memory's max_time).

Returns
alignments Tensor of dtype matching self.values and shape [batch_size, alignments_size] (alignments_size is memory's max_time).