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transformer.py
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from utils import *
class MultiHeadAttention(nn.Module):
def __init__(self, d_model, num_heads, dropout=0., gain=1.):
super().__init__()
assert d_model % num_heads == 0, "d_model must be divisible by num_heads"
self.d_model = d_model
self.num_heads = num_heads
self.attn_dropout = nn.Dropout(dropout)
self.output_dropout = nn.Dropout(dropout)
self.proj_q = linear(d_model, d_model, bias=False)
self.proj_k = linear(d_model, d_model, bias=False)
self.proj_v = linear(d_model, d_model, bias=False)
self.proj_o = linear(d_model, d_model, bias=False, gain=gain)
def forward(self, q, k, v, attn_mask=None):
"""
q: batch_size x target_len x d_model
k: batch_size x source_len x d_model
v: batch_size x source_len x d_model
attn_mask: target_len x source_len
return: batch_size x target_len x d_model
"""
B, T, _ = q.shape
_, S, _ = k.shape
q = self.proj_q(q).view(B, T, self.num_heads, -1).transpose(1, 2)
k = self.proj_k(k).view(B, S, self.num_heads, -1).transpose(1, 2)
v = self.proj_v(v).view(B, S, self.num_heads, -1).transpose(1, 2)
q = q * (q.shape[-1] ** (-0.5))
attn = torch.matmul(q, k.transpose(-1, -2))
if attn_mask is not None:
attn = attn.masked_fill(attn_mask, float('-inf'))
attn = F.softmax(attn, dim=-1)
attn = self.attn_dropout(attn)
output = torch.matmul(attn, v).transpose(1, 2).reshape(B, T, -1)
output = self.proj_o(output)
output = self.output_dropout(output)
return output
class TransformerEncoderBlock(nn.Module):
def __init__(self, d_model, num_heads, dropout=0., gain=1., is_first=False):
super().__init__()
self.is_first = is_first
self.attn_layer_norm = nn.LayerNorm(d_model)
self.attn = MultiHeadAttention(d_model, num_heads, dropout, gain)
self.ffn_layer_norm = nn.LayerNorm(d_model)
self.ffn = nn.Sequential(
linear(d_model, 4 * d_model, weight_init='kaiming'),
nn.ReLU(),
linear(4 * d_model, d_model, gain=gain),
nn.Dropout(dropout))
def forward(self, input):
"""
input: batch_size x source_len x d_model
return: batch_size x source_len x d_model
"""
if self.is_first:
input = self.attn_layer_norm(input)
x = self.attn(input, input, input)
input = input + x
else:
x = self.attn_layer_norm(input)
x = self.attn(x, x, x)
input = input + x
x = self.ffn_layer_norm(input)
x = self.ffn(x)
return input + x
class TransformerEncoder(nn.Module):
def __init__(self, num_blocks, d_model, num_heads, dropout=0.):
super().__init__()
if num_blocks > 0:
gain = (2 * num_blocks) ** (-0.5)
self.blocks = nn.ModuleList(
[TransformerEncoderBlock(d_model, num_heads, dropout, gain, is_first=True)] +
[TransformerEncoderBlock(d_model, num_heads, dropout, gain, is_first=False)
for _ in range(num_blocks - 1)])
else:
self.blocks = nn.ModuleList()
self.layer_norm = nn.LayerNorm(d_model)
def forward(self, input):
"""
input: batch_size x source_len x d_model
return: batch_size x source_len x d_model
"""
for block in self.blocks:
input = block(input)
return self.layer_norm(input)
class TransformerDecoderBlock(nn.Module):
def __init__(self, max_len, d_model, num_heads, dropout=0., gain=1., is_first=False):
super().__init__()
self.is_first = is_first
self.self_attn_layer_norm = nn.LayerNorm(d_model)
self.self_attn = MultiHeadAttention(d_model, num_heads, dropout, gain)
mask = torch.triu(torch.ones((max_len, max_len), dtype=torch.bool), diagonal=1)
self.self_attn_mask = nn.Parameter(mask, requires_grad=False)
self.encoder_decoder_attn_layer_norm = nn.LayerNorm(d_model)
self.encoder_decoder_attn = MultiHeadAttention(d_model, num_heads, dropout, gain)
self.ffn_layer_norm = nn.LayerNorm(d_model)
self.ffn = nn.Sequential(
linear(d_model, 4 * d_model, weight_init='kaiming'),
nn.ReLU(),
linear(4 * d_model, d_model, gain=gain),
nn.Dropout(dropout))
def forward(self, input, encoder_output):
"""
input: batch_size x target_len x d_model
encoder_output: batch_size x source_len x d_model
return: batch_size x target_len x d_model
"""
T = input.shape[1]
if self.is_first:
input = self.self_attn_layer_norm(input)
x = self.self_attn(input, input, input, self.self_attn_mask[:T, :T])
input = input + x
else:
x = self.self_attn_layer_norm(input)
x = self.self_attn(x, x, x, self.self_attn_mask[:T, :T])
input = input + x
x = self.encoder_decoder_attn_layer_norm(input)
x = self.encoder_decoder_attn(x, encoder_output, encoder_output)
input = input + x
x = self.ffn_layer_norm(input)
x = self.ffn(x)
return input + x
class TransformerDecoder(nn.Module):
def __init__(self, num_blocks, max_len, d_model, num_heads, dropout=0.):
super().__init__()
if num_blocks > 0:
gain = (3 * num_blocks) ** (-0.5)
self.blocks = nn.ModuleList(
[TransformerDecoderBlock(max_len, d_model, num_heads, dropout, gain, is_first=True)] +
[TransformerDecoderBlock(max_len, d_model, num_heads, dropout, gain, is_first=False)
for _ in range(num_blocks - 1)])
else:
self.blocks = nn.ModuleList()
self.layer_norm = nn.LayerNorm(d_model)
def forward(self, input, encoder_output):
"""
input: batch_size x target_len x d_model
encoder_output: batch_size x source_len x d_model
return: batch_size x target_len x d_model
"""
for block in self.blocks:
input = block(input, encoder_output)
return self.layer_norm(input)