tf.contrib.seq2seq.BahdanauMonotonicAttention

Monotonic attention mechanism with Bahadanau-style energy function.

tf.contrib.seq2seq.BahdanauMonotonicAttention(
    num_units, memory, memory_sequence_length=None, normalize=False,
    score_mask_value=None, sigmoid_noise=0.0, sigmoid_noise_seed=None,
    score_bias_init=0.0, mode='parallel', dtype=None,
    name='BahdanauMonotonicAttention'
)

This type of attention enforces a monotonic constraint on the attention distributions; that is once the model attends to a given point in the memory it can't attend to any prior points at subsequence output timesteps. It achieves this by using the _monotonic_probability_fn instead of softmax to construct its attention distributions. Since the attention scores are passed through a sigmoid, a learnable scalar bias parameter is applied after the score function and before the sigmoid. Otherwise, it is equivalent to BahdanauAttention. This approach is proposed in

Colin Raffel, Minh-Thang Luong, Peter J. Liu, Ron J. Weiss, Douglas Eck, "Online and Linear-Time Attention by Enforcing Monotonic Alignments." ICML 2017. https://arxiv.org/abs/1704.00784

Args
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.
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.
sigmoid_noise	Standard deviation of pre-sigmoid noise. See the docstring for _monotonic_probability_fn for more information.
sigmoid_noise_seed	(optional) Random seed for pre-sigmoid noise.
score_bias_init	Initial value for score bias scalar. It's recommended to initialize this to a negative value when the length of the memory is large.
mode	How to compute the attention distribution. Must be one of 'recursive', 'parallel', or 'hard'. See the docstring for tf.contrib.seq2seq.monotonic_attention for more information.
dtype	The data type for the query and memory layers of the attention mechanism.
name	Name to use when creating ops.

Attributes
alignments_size
batch_size
keys
memory_layer
query_layer
state_size
values

Methods

initial_alignments

View source

initial_alignments(
    batch_size, dtype
)

Creates the initial alignment values for the monotonic attentions.

Initializes to dirac distributions, i.e. [1, 0, 0, ...memory length..., 0] for all entries in the batch.

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

View source

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__

View source

__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).