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.

When using MultiHeadAttention inside a custom layer, the custom layer must implement its own build() method and call MultiHeadAttention's _build_from_signature() there. This enables weights to be restored correctly when the model is loaded.

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.

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, attention_scores) if True, or attention_output 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.
use_causal_mask A boolean to indicate whether to apply a causal mask to prevent tokens from attending to future tokens (e.g., used in a decoder Transformer).

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 projected to the shape specified by output_shape.
attention_scores [Optional] multi-head attention coefficients over attention axes.