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"""
Implementation of "Attention is All You Need"
"""
import torch.nn as nn
from onmt.encoders.encoder import EncoderBase
from onmt.modules import MultiHeadedAttention
from onmt.modules.position_ffn import PositionwiseFeedForward
from onmt.modules.position_ffn import ActivationFunction
from onmt.utils.misc import sequence_mask
class TransformerEncoderLayer(nn.Module):
"""
A single layer of the transformer encoder.
Args:
d_model (int): the dimension of keys/values/queries in
MultiHeadedAttention, also the input size of
the first-layer of the PositionwiseFeedForward.
heads (int): the number of head for MultiHeadedAttention.
d_ff (int): the second-layer of the PositionwiseFeedForward.
dropout (float): dropout probability(0-1.0).
pos_ffn_activation_fn (ActivationFunction):
activation function choice for PositionwiseFeedForward layer
"""
def __init__(self, d_model, heads, d_ff, dropout, attention_dropout,
max_relative_positions=0,
pos_ffn_activation_fn=ActivationFunction.relu):
super(TransformerEncoderLayer, self).__init__()
self.self_attn = MultiHeadedAttention(
heads, d_model, dropout=attention_dropout,
max_relative_positions=max_relative_positions)
self.feed_forward = PositionwiseFeedForward(d_model, d_ff, dropout,
pos_ffn_activation_fn)
self.layer_norm = nn.LayerNorm(d_model, eps=1e-6)
self.dropout = nn.Dropout(dropout)
def forward(self, inputs, mask):
"""
Args:
inputs (FloatTensor): ``(batch_size, src_len, model_dim)``
mask (LongTensor): ``(batch_size, 1, src_len)``
Returns:
(FloatTensor):
* outputs ``(batch_size, src_len, model_dim)``
"""
input_norm = self.layer_norm(inputs)
context, _ = self.self_attn(input_norm, input_norm, input_norm,
mask=mask, attn_type="self")
out = self.dropout(context) + inputs
return self.feed_forward(out)
def update_dropout(self, dropout, attention_dropout):
self.self_attn.update_dropout(attention_dropout)
self.feed_forward.update_dropout(dropout)
self.dropout.p = dropout
class TransformerEncoder(EncoderBase):
"""The Transformer encoder from "Attention is All You Need"
:cite:`DBLP:journals/corr/VaswaniSPUJGKP17`
.. mermaid::
graph BT
A[input]
B[multi-head self-attn]
C[feed forward]
O[output]
A --> B
B --> C
C --> O
Args:
num_layers (int): number of encoder layers
d_model (int): size of the model
heads (int): number of heads
d_ff (int): size of the inner FF layer
dropout (float): dropout parameters
embeddings (onmt.modules.Embeddings):
embeddings to use, should have positional encodings
pos_ffn_activation_fn (ActivationFunction):
activation function choice for PositionwiseFeedForward layer
Returns:
(torch.FloatTensor, torch.FloatTensor):
* embeddings ``(src_len, batch_size, model_dim)``
* memory_bank ``(src_len, batch_size, model_dim)``
"""
def __init__(self, num_layers, d_model, heads, d_ff, dropout,
attention_dropout, embeddings, max_relative_positions,
pos_ffn_activation_fn=ActivationFunction.relu):
super(TransformerEncoder, self).__init__()
self.embeddings = embeddings
self.transformer = nn.ModuleList(
[TransformerEncoderLayer(
d_model, heads, d_ff, dropout, attention_dropout,
max_relative_positions=max_relative_positions,
pos_ffn_activation_fn=pos_ffn_activation_fn)
for i in range(num_layers)])
self.layer_norm = nn.LayerNorm(d_model, eps=1e-6)
@classmethod
def from_opt(cls, opt, embeddings):
"""Alternate constructor."""
return cls(
opt.enc_layers,
opt.enc_rnn_size,
opt.heads,
opt.transformer_ff,
opt.dropout[0] if type(opt.dropout) is list else opt.dropout,
opt.attention_dropout[0] if type(opt.attention_dropout)
is list else opt.attention_dropout,
embeddings,
opt.max_relative_positions,
pos_ffn_activation_fn=opt.pos_ffn_activation_fn,
)
def forward(self, src, lengths=None):
"""See :func:`EncoderBase.forward()`"""
self._check_args(src, lengths)
emb = self.embeddings(src)
out = emb.transpose(0, 1).contiguous()
mask = ~sequence_mask(lengths).unsqueeze(1)
# Run the forward pass of every layer of the tranformer.
for layer in self.transformer:
out = layer(out, mask)
out = self.layer_norm(out)
return emb, out.transpose(0, 1).contiguous(), lengths
def update_dropout(self, dropout, attention_dropout):
self.embeddings.update_dropout(dropout)
for layer in self.transformer:
layer.update_dropout(dropout, attention_dropout)
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