R/layer-attention.R

layer_multi_head_attention

MultiHeadAttention layer

Description

This is an implementation of multi-headed attention based on “Attention is all you Need”. If query, key, value are the same, then this is self-attention. Each timestep in query attends to the corresponding sequence in key, and returns a fixed-width vector.

Usage

 
layer_multi_head_attention( 
  inputs, 
  num_heads, 
  key_dim, 
  value_dim = NULL, 
  dropout = 0, 
  use_bias = TRUE, 
  output_shape = NULL, 
  attention_axes = NULL, 
  kernel_initializer = "glorot_uniform", 
  bias_initializer = "zeros", 
  kernel_regularizer = NULL, 
  bias_regularizer = NULL, 
  activity_regularizer = NULL, 
  kernel_constraint = NULL, 
  bias_constraint = NULL, 
  ... 
)

Arguments

Arguments Description
inputs a list of inputs first should be the query tensor, the second the value tensor
num_heads Number of attention heads.
key_dim Size of each attention head for query and key.
value_dim Size of each attention head for value.
dropout Dropout probability.
use_bias Boolean, whether the dense layers use bias vectors/matrices.
output_shape The expected shape of an output tensor, besides the batch and sequence dims. If not specified, projects back to the key feature dim.
attention_axes axes over which the attention is applied. None means attention over all axes, but batch, heads, and features.
kernel_initializer Initializer for dense layer kernels.
bias_initializer Initializer for dense layer biases.
kernel_regularizer Regularizer for dense layer kernels.
bias_regularizer Regularizer for dense layer biases.
activity_regularizer Regularizer for dense layer activity.
kernel_constraint Constraint for dense layer kernels.
bias_constraint Constraint for dense layer kernels.
… Other arguments passed to the layer. Eg, name, training.

Details

This layer first projects query, key and value. These are (effectively) a list of tensors of length num_attention_heads, where the corresponding shapes are [batch_size, , key_dim], [batch_size, , key_dim], [batch_size, , value_dim]. Then, the query and key tensors are dot-producted and scaled. These are softmaxed to obtain attention probabilities. The value tensors are then interpolated by these probabilities, then concatenated back to a single tensor. Finally, the result tensor with the last dimension as value_dim can take an linear projection and return.

Section

Call arguments

  • query: Query Tensor of shape [B, T, dim].

  • value: Value Tensor of shape [B, S, dim].

  • key: Optional key Tensor of shape [B, S, dim]. If not given, will use value for both key and value, which is the most common case.

  • attention_mask: a boolean mask of shape [B, T, S], that prevents attention to certain positions.

  • return_attention_scores: A boolean to indicate whether the output should be attention output if TRUE, or (attention_output, attention_scores) if FALSE. Defaults to FALSE.

  • training: Python boolean indicating whether the layer should behave in training mode (adding dropout) or in inference mode (no dropout). Defaults to either using the training mode of the parent layer/model, or FALSE (inference) if there is no parent layer.

Value

  • attention_output: The result of the computation, of shape [B, T, E], where T is for target sequence shapes and E is the query input last dimension if output_shape is None. Otherwise, the multi-head outputs are project to the shape specified by output_shape.

  • attention_scores: (Optional) multi-head attention coeffients over attention axes.