TransformerDecoder

class paddle.nn. TransformerDecoder ( decoder_layer, num_layers, norm=None ) [source]

TransformerDecoder is a stack of N decoder layers.

Parameters
  • decoder_layer (Layer) – an instance of the TransformerDecoderLayer. It would be used as the first layer, and the other layers would be created according to the configurations of it.

  • num_layers (int) – The number of decoder layers to be stacked.

  • norm (LayerNorm, optional) – the layer normalization component. If provided, apply layer normalization on the output of last encoder layer.

Examples

import paddle
from paddle.nn import TransformerDecoderLayer, TransformerDecoder

# decoder input: [batch_size, tgt_len, d_model]
dec_input = paddle.rand((2, 4, 128))
# encoder output: [batch_size, src_len, d_model]
enc_output = paddle.rand((2, 6, 128))
# self attention mask: [batch_size, n_head, tgt_len, tgt_len]
self_attn_mask = paddle.rand((2, 2, 4, 4))
# cross attention mask: [batch_size, n_head, tgt_len, src_len]
cross_attn_mask = paddle.rand((2, 2, 4, 6))
decoder_layer = TransformerDecoderLayer(128, 2, 512)
decoder = TransformerDecoder(decoder_layer, 2)
output = decoder(dec_input,
                 enc_output,
                 self_attn_mask,
                 cross_attn_mask)  # [2, 4, 128]
forward ( tgt, memory, tgt_mask=None, memory_mask=None, cache=None )

Applies a stack of N Transformer decoder layers on inputs. If norm is provided, also applies layer normalization on the output of last decoder layer.

Parameters
  • tgt (Tensor) – The input of Transformer decoder. It is a tensor with shape [batch_size, target_length, d_model]. The data type should be float32 or float64.

  • memory (Tensor) – The output of Transformer encoder. It is a tensor with shape [batch_size, source_length, d_model]. The data type should be float32 or float64.

  • tgt_mask (Tensor, optional) – A tensor used in self attention to prevents attention to some unwanted positions, usually the the subsequent positions. It is a tensor with shape broadcasted to [batch_size, n_head, target_length, target_length], where the unwanted positions have -INF values and the others have 0 values. The data type should be float32 or float64. It can be None when nothing wanted or needed to be prevented attention to. Default None

  • memory_mask (Tensor, optional) –

    A tensor used in decoder-encoder

    cross attention to prevents attention to some unwanted positions, usually the paddings. It is a tensor with shape broadcasted to

    [batch_size, n_head, target_length, source_length], where the

    unwanted positions have -INF values and the others have 0 values. The data type should be float32 or float64. It can be None when nothing wanted or needed to be prevented attention to. Default None

  • cache (list, optional) – It is a list, and each element in the list is a tuple( (incremental_cache, static_cache) ). See TransformerDecoder.gen_cache for more details. It is only used for inference and should be None for training. Default None.

Returns

It is a tensor that has the same shape and data type

as tgt, representing the output of Transformer decoder. Or a tuple if cache is not None, except for decoder output, the tuple includes the new cache which is same as input cache argument but incremental_cache in it has an incremental length. See MultiHeadAttention.gen_cache and MultiHeadAttention.forward for more details.

Return type

Tensor|tuple

gen_cache ( memory, do_zip=False )

Generates cache for forward usage. The generated cache is a list, and each element in it is a tuple( (incremental_cache, static_cache) ) produced by TransformerDecoderLayer.gen_cache. See TransformerDecoderLayer.gen_cache for more details. If do_zip is True, apply zip on these tuples to get a list with two elements.

Parameters
  • memory (Tensor) – The output of Transformer encoder. It is a tensor with shape [batch_size, source_length, d_model]. The data type should be float32 or float64.

  • do_zip (bool, optional) – Indicate whether to apply zip on the tuples. If True, return a list with two elements. Default False

Returns

It is a list, and each element in the list is a tuple produced

by TransformerDecoderLayer.gen_cache(memory). See TransformerDecoderLayer.gen_cache for more details. If do_zip is True, apply zip on these tuples and return a list with two elements.

Return type

list