attention.py

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function
from __future__ import unicode_literals


class AttentionType:
    Regular, Recurrent = range(2)


def s(scope, name):
    # We have to manually scope due to our internal/external blob
    # relationships.
    return "{}/{}".format(str(scope), str(name))


# c_i = \sum_j w_{ij}\textbf{s}_j
def _calc_weighted_context(
    model,
    encoder_outputs_transposed,
    encoder_output_dim,
    attention_weights_3d,
    scope,
):
    # [batch_size, encoder_output_dim, 1]
    attention_weighted_encoder_context = model.net.BatchMatMul(
        [encoder_outputs_transposed, attention_weights_3d],
        s(scope, 'attention_weighted_encoder_context'),
    )
    # TODO: somehow I cannot use Squeeze in-place op here
    # [batch_size, encoder_output_dim]
    attention_weighted_encoder_context, _ = model.net.Reshape(
        attention_weighted_encoder_context,
        [
            attention_weighted_encoder_context,
            s(scope, 'attention_weighted_encoder_context_old_shape')
        ],
        shape=[1, -1, encoder_output_dim],
    )
    return attention_weighted_encoder_context


# Calculate a softmax over the passed in attention energy logits
def _calc_attention_weights(
    model,
    attention_logits_transposed,
    scope
):
    # TODO: we could try to force some attention weights to be zeros,
    # based on encoder_lengths.
    # [batch_size, encoder_length]
    attention_weights = model.Softmax(
        attention_logits_transposed,
        s(scope, 'attention_weights'),
        engine='CUDNN',
    )
    # TODO: make this operation in-place
    # [batch_size, encoder_length, 1]
    attention_weights_3d = model.net.ExpandDims(
        attention_weights,
        s(scope, 'attention_weights_3d'),
        dims=[2],
    )
    return attention_weights_3d


# e_{ij} = \textbf{v}^T tanh \alpha(\textbf{h}_{i-1}, \textbf{s}_j)
def _calc_attention_logits_from_sum_match(
    model,
    decoder_hidden_encoder_outputs_sum,
    encoder_output_dim,
    scope
):
    # [encoder_length, batch_size, encoder_output_dim]
    decoder_hidden_encoder_outputs_sum = model.net.Tanh(
        decoder_hidden_encoder_outputs_sum,
        decoder_hidden_encoder_outputs_sum,
    )

    attention_v = model.param_init_net.XavierFill(
        [],
        s(scope, 'attention_v'),
        shape=[1, encoder_output_dim],
    )
    model.add_param(attention_v)

    attention_zeros = model.param_init_net.ConstantFill(
        [],
        s(scope, 'attention_zeros'),
        value=0.0,
        shape=[1],
    )

    # [encoder_length, batch_size, 1]
    attention_logits = model.net.FC(
        [decoder_hidden_encoder_outputs_sum, attention_v, attention_zeros],
        [s(scope, 'attention_logits')],
        axis=2
    )
    # [encoder_length, batch_size]
    attention_logits = model.net.Squeeze(
        [attention_logits],
        [attention_logits],
        dims=[2],
    )
    # [batch_size, encoder_length]
    attention_logits_transposed = model.Transpose(
        attention_logits,
        s(scope, 'attention_logits_transposed'),
        axes=[1, 0],
    )
    return attention_logits_transposed


# \textbf{W}^\alpha used in the context of \alpha_{sum}(a,b)
def _apply_fc_weight_for_sum_match(
    model,
    input,
    dim_in,
    dim_out,
    scope,
    name
):
    output = model.FC(
        input,
        s(scope, name),
        dim_in=dim_in,
        dim_out=dim_out,
        axis=2,
    )
    output = model.net.Squeeze(
        output,
        output,
        dims=[0]
    )
    return output


# Implement RecAtt due to section 4.1 in http://arxiv.org/abs/1601.03317
def apply_recurrent_attention(
    model,
    encoder_output_dim,
    encoder_outputs_transposed,
    weighted_encoder_outputs,
    decoder_hidden_state_t,
    decoder_hidden_state_dim,
    attention_weighted_encoder_context_t_prev,
    scope,
):
    weighted_prev_attention_context = _apply_fc_weight_for_sum_match(
        model=model,
        input=attention_weighted_encoder_context_t_prev,
        dim_in=encoder_output_dim,
        dim_out=encoder_output_dim,
        scope=scope,
        name='weighted_prev_attention_context'
    )

    weighted_decoder_hidden_state = _apply_fc_weight_for_sum_match(
        model=model,
        input=decoder_hidden_state_t,
        dim_in=decoder_hidden_state_dim,
        dim_out=encoder_output_dim,
        scope=scope,
        name='weighted_decoder_hidden_state'
    )

    # [encoder_length, batch_size, encoder_output_dim]
    decoder_hidden_encoder_outputs_sum_tmp = model.net.Add(
        [
            weighted_encoder_outputs,
            weighted_decoder_hidden_state
        ],
        s(scope, 'decoder_hidden_encoder_outputs_sum_tmp'),
        broadcast=1,
        use_grad_hack=1,
    )
    # [encoder_length, batch_size, encoder_output_dim]
    decoder_hidden_encoder_outputs_sum = model.net.Add(
        [
            decoder_hidden_encoder_outputs_sum_tmp,
            weighted_prev_attention_context
        ],
        s(scope, 'decoder_hidden_encoder_outputs_sum'),
        broadcast=1,
        use_grad_hack=1,
    )

    attention_logits_transposed = _calc_attention_logits_from_sum_match(
        model=model,
        decoder_hidden_encoder_outputs_sum=decoder_hidden_encoder_outputs_sum,
        encoder_output_dim=encoder_output_dim,
        scope=scope
    )

    # [batch_size, encoder_length, 1]
    attention_weights_3d = _calc_attention_weights(
        model=model,
        attention_logits_transposed=attention_logits_transposed,
        scope=scope
    )

    # [batch_size, encoder_output_dim, 1]
    attention_weighted_encoder_context = _calc_weighted_context(
        model=model,
        encoder_outputs_transposed=encoder_outputs_transposed,
        encoder_output_dim=encoder_output_dim,
        attention_weights_3d=attention_weights_3d,
        scope=scope
    )
    return attention_weighted_encoder_context, attention_weights_3d, [
        decoder_hidden_encoder_outputs_sum_tmp,
        decoder_hidden_encoder_outputs_sum
    ]


def apply_regular_attention(
    model,
    encoder_output_dim,
    encoder_outputs_transposed,
    weighted_encoder_outputs,
    decoder_hidden_state_t,
    decoder_hidden_state_dim,
    scope,
):
    weighted_decoder_hidden_state = _apply_fc_weight_for_sum_match(
        model=model,
        input=decoder_hidden_state_t,
        dim_in=decoder_hidden_state_dim,
        dim_out=encoder_output_dim,
        scope=scope,
        name='weighted_decoder_hidden_state'
    )

    # [encoder_length, batch_size, encoder_output_dim]
    decoder_hidden_encoder_outputs_sum = model.net.Add(
        [weighted_encoder_outputs, weighted_decoder_hidden_state],
        s(scope, 'decoder_hidden_encoder_outputs_sum'),
        broadcast=1,
        use_grad_hack=1,
    )

    attention_logits_transposed = _calc_attention_logits_from_sum_match(
        model=model,
        decoder_hidden_encoder_outputs_sum=decoder_hidden_encoder_outputs_sum,
        encoder_output_dim=encoder_output_dim,
        scope=scope
    )

    # [batch_size, encoder_length, 1]
    attention_weights_3d = _calc_attention_weights(
        model=model,
        attention_logits_transposed=attention_logits_transposed,
        scope=scope
    )

    # [batch_size, encoder_output_dim, 1]
    attention_weighted_encoder_context = _calc_weighted_context(
        model=model,
        encoder_outputs_transposed=encoder_outputs_transposed,
        encoder_output_dim=encoder_output_dim,
        attention_weights_3d=attention_weights_3d,
        scope=scope
    )
    return attention_weighted_encoder_context, attention_weights_3d, [
        decoder_hidden_encoder_outputs_sum
    ]