Dynamic attention in tensorflow

Posted 2020-08-29 游园惊梦(https://github.com/chengh

新代码在contrib\\seq2seq\\python\\ops\\attention_decoder_fn.py

   

和之前代码相比 不再采用conv的方式来计算乘，直接使用乘法和linear

   

给出了两种attention的实现 传统的"bahdanau": additive (Bahdanau et al., ICLR\'2015) Neural Machine Translation by Jointly Learning to Align and Translate

以及"luong": multiplicative (Luong et al., EMNLP\'2015) Effective Approaches to Attention-based Neural Machine Translation

   

这里以 bahdanau为例

   

还是按照 Grammar as a Foreign Language的公式

   

对应代码里面

将input encoder outputs 也就是输入的attention states作为 attention values

   

也就是在prepare_attention中

attention_values = attention_states

那么attention keys 对应 W_1h_i的部分，采用linear来实现

attention_keys = layers.linear(

attention_states, num_units, biases_initializer=None, scope=scope)

   

在创建score function的

_create_attention_score_fn 中完整定义了计算过程

这里去掉luong的实现部分 仅仅看bahdanau部分

   

with variable_scope.variable_scope(name, reuse=reuse):

if attention_option == "bahdanau":

#这里对应第一个公式最右面 query_w对应W_2, query是对应d_t

query_w = variable_scope.get_variable(

"attnW", [num_units, num_units], dtype=dtype)

#对应第一个公式最左侧的v

score_v = variable_scope.get_variable("attnV", [num_units], dtype=dtype)

   

def attention_score_fn(query, keys, values):

"""Put attention masks on attention_values using attention_keys and query.

   

Args:

query: A Tensor of shape [batch_size, num_units].

keys: A Tensor of shape [batch_size, attention_length, num_units].

values: A Tensor of shape [batch_size, attention_length, num_units].

   

Returns:

context_vector: A Tensor of shape [batch_size, num_units].

   

Raises:

ValueError: if attention_option is neither "luong" or "bahdanau".

   

   

"""

if attention_option == "bahdanau":

# transform query W_2*d_t

query = math_ops.matmul(query, query_w)

   

# reshape query: [batch_size, 1, num_units]

query = array_ops.reshape(query, [-1, 1, num_units])

   

# attn_fun 对应第一个公式的最左侧结果(=左侧) math_ops.reduce_sum(v * math_ops.tanh(keys + query), [2]) * + reduce_sum操作即是dot操作

scores = _attn_add_fun(score_v, keys, query)

   

# Compute alignment weights

# scores: [batch_size, length]

# alignments: [batch_size, length]

# TODO(thangluong): not normalize over padding positions.

#对应第二个公式计算softmax结果

alignments = nn_ops.softmax(scores)

   

# Now calculate the attention-weighted vector.

alignments = array_ops.expand_dims(alignments, 2)

#利用softmax得到的权重 计算attention向量的加权加和

context_vector = math_ops.reduce_sum(alignments * values, [1])

context_vector.set_shape([None, num_units])

   

#context_vector即对应 第三个公式 =的左侧

return context_vector

 

再看下计算出contenxt_vector之后的使用，这个方法正如论文中所说也和之前旧代码基本一致

也就是说将context和query进行concat之后通过linear映射依然得到num_units的长度 作为attention

def _create_attention_construct_fn(name, num_units, attention_score_fn, reuse):

"""Function to compute attention vectors.

   

Args:

name: to label variables.

num_units: hidden state dimension.

attention_score_fn: to compute similarity between key and target states.

reuse: whether to reuse variable scope.

   

Returns:

attention_construct_fn: to build attention states.

"""

with variable_scope.variable_scope(name, reuse=reuse) as scope:

   

def construct_fn(attention_query, attention_keys, attention_values):

context = attention_score_fn(attention_query, attention_keys,

attention_values)

concat_input = array_ops.concat([attention_query, context], 1)

attention = layers.linear(

concat_input, num_units, biases_initializer=None, scope=scope)

return attention

   

return construct_fn

   

   

   

最终的使用，cell_output就是attention，而next_input是cell_input和attention的concat

# construct attention

attention = attention_construct_fn(cell_output, attention_keys,

attention_values)

cell_output = attention

   

# argmax decoder

cell_output = output_fn(cell_output) # logits

next_input_id = math_ops.cast(

math_ops.argmax(cell_output, 1), dtype=dtype)

done = math_ops.equal(next_input_id, end_of_sequence_id)

cell_input = array_ops.gather(embeddings, next_input_id)

   

# combine cell_input and attention

next_input = array_ops.concat([cell_input, attention], 1)