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113 lines (99 loc) · 3.86 KB
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import math
import torch
class PositionalEncoding(torch.nn.Module):
"""Positional encoding.
Args:
d_model (int): Embedding dimension.
dropout_rate (float): Dropout rate.
max_len (int): Maximum input length.
reverse (bool): Whether to reverse the input position.
"""
def __init__(self, d_model, dropout_rate, max_len=5000, reverse=False):
"""Construct an PositionalEncoding object."""
super(PositionalEncoding, self).__init__()
self.d_model = d_model
self.reverse = reverse
self.xscale = math.sqrt(self.d_model)
self.dropout = torch.nn.Dropout(p=dropout_rate)
self.pe = None
self.extend_pe(torch.tensor(0.0).expand(1, max_len))
def extend_pe(self, x):
"""Reset the positional encodings."""
if self.pe is not None:
if self.pe.size(1) >= x.size(1):
if self.pe.dtype != x.dtype or self.pe.device != x.device:
self.pe = self.pe.to(dtype=x.dtype, device=x.device)
return
if self.reverse:
position = torch.arange(
x.size(1) - 1, -1, -1.0, dtype=torch.float32
).unsqueeze(1)
else:
position = torch.arange(0, x.size(1), dtype=torch.float32).unsqueeze(1)
div_term = torch.exp(
torch.arange(0, self.d_model, 2, dtype=torch.float32)
* -(math.log(10000.0) / self.d_model)
)
pe = torch.stack([
torch.sin(position * div_term),
torch.cos(position * div_term)
], dim=2).view(-1, self.d_model).unsqueeze(0)
self.pe = pe.to(device=x.device, dtype=x.dtype)
def forward(self, x: torch.Tensor):
"""Add positional encoding.
Args:
x (torch.Tensor): Input tensor (batch, time, `*`).
Returns:
torch.Tensor: Encoded tensor (batch, time, `*`).
"""
self.extend_pe(x)
x = x * self.xscale + self.pe[:, : x.size(1)]
return self.dropout(x)
class ScaledPositionalEncoding(PositionalEncoding):
"""Scaled positional encoding module.
See Sec. 3.2 https://arxiv.org/abs/1809.08895
Args:
d_model (int): Embedding dimension.
dropout_rate (float): Dropout rate.
max_len (int): Maximum input length.
"""
def __init__(self, d_model, dropout_rate, max_len=5000):
"""Initialize class."""
super().__init__(d_model=d_model, dropout_rate=dropout_rate, max_len=max_len)
self.alpha = torch.nn.Parameter(torch.tensor(1.0))
def reset_parameters(self):
"""Reset parameters."""
self.alpha.data = torch.tensor(1.0)
def forward(self, x):
"""Add positional encoding.
Args:
x (torch.Tensor): Input tensor (batch, time, `*`).
Returns:
torch.Tensor: Encoded tensor (batch, time, `*`).
"""
self.extend_pe(x)
x = x + self.alpha * self.pe[:, : x.size(1)]
return self.dropout(x)
class RelPositionalEncoding(PositionalEncoding):
"""Relative positional encoding module.
See : Appendix B in https://arxiv.org/abs/1901.02860
Args:
d_model (int): Embedding dimension.
dropout_rate (float): Dropout rate.
max_len (int): Maximum input length.
"""
def __init__(self, d_model, dropout_rate, max_len=5000):
"""Initialize class."""
super().__init__(d_model, dropout_rate, max_len, reverse=True)
def forward(self, x):
"""Compute positional encoding.
Args:
x (torch.Tensor): Input tensor (batch, time, `*`).
Returns:
torch.Tensor: Encoded tensor (batch, time, `*`).
torch.Tensor: Positional embedding tensor (1, time, `*`).
"""
self.extend_pe(x)
x = x * self.xscale
pos_emb = self.pe[:, : x.size(1)]
return self.dropout(x) + self.dropout(pos_emb)