tf.keras.layers.MultiHeadAttention

MultiHeadAttention layer.

Inherits From: Layer, Module

tf.keras.layers.MultiHeadAttention(
    num_heads, key_dim, value_dim=None, dropout=0.0, use_bias=True,
    output_shape=None, attention_axes=None,
    kernel_initializer='glorot_uniform',
    bias_initializer='zeros', kernel_regularizer=None,
    bias_regularizer=None, activity_regularizer=None, kernel_constraint=None,
    bias_constraint=None, **kwargs
)

This is an implementation of multi-headed attention as described in the paper "Attention is all you Need" (Vaswani et al., 2017). 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.

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, <query dimensions>, key_dim), (batch_size, <key/value dimensions>, key_dim), (batch_size, <key/value dimensions>, 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.

Examples:

Performs 1D cross-attention over two sequence inputs with an attention mask. Returns the additional attention weights over heads.

layer = MultiHeadAttention(num_heads=2, key_dim=2)
target = tf.keras.Input(shape=[8, 16])
source = tf.keras.Input(shape=[4, 16])
output_tensor, weights = layer(target, source,
                               return_attention_scores=True)
print(output_tensor.shape)
(None, 8, 16)
print(weights.shape)
(None, 2, 8, 4)

Performs 2D self-attention over a 5D input tensor on axes 2 and 3.

layer = MultiHeadAttention(num_heads=2, key_dim=2, attention_axes=(2, 3))
input_tensor = tf.keras.Input(shape=[5, 3, 4, 16])
output_tensor = layer(input_tensor, input_tensor)
print(output_tensor.shape)
(None, 5, 3, 4, 16)

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.

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. The boolean mask specifies which query elements can attend to which key elements, 1 indicates attention and 0 indicates no attention. Broadcasting can happen for the missing batch dimensions and the head dimension.
  • 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.

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.