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import torch
import torch.nn as nn
import math
class LayerNormalization(nn.Module):
def __init__(self, features: int, eps:float=10**-6) -> None:
super().__init__()
self.eps = eps
self.alpha = nn.Parameter(torch.ones(features)) # alpha is a learnable parameter
self.bias = nn.Parameter(torch.zeros(features)) # bias is a learnable parameter
def forward(self, x):
# x: (batch, seq_len, hidden_size)
# Keep the dimension for broadcasting
mean = x.mean(dim = -1, keepdim = True) # (batch, seq_len, 1)
# Keep the dimension for broadcasting
std = x.std(dim = -1, keepdim = True) # (batch, seq_len, 1)
# eps is to prevent dividing by zero or when std is very small
return self.alpha * (x - mean) / (std + self.eps) + self.bias
class FeedForwardBlock(nn.Module):
def __init__(self, d_model: int, d_ff: int, dropout: float) -> None:
super().__init__()
self.linear_1 = nn.Linear(d_model, d_ff) # w1 and b1
self.dropout = nn.Dropout(dropout)
self.linear_2 = nn.Linear(d_ff, d_model) # w2 and b2
def forward(self, x):
# (batch, seq_len, d_model) --> (batch, seq_len, d_ff) --> (batch, seq_len, d_model)
return self.linear_2(self.dropout(torch.relu(self.linear_1(x))))
class InputEmbeddings(nn.Module):
def __init__(self, d_model: int, vocab_size: int) -> None:
super().__init__()
self.d_model = d_model
self.vocab_size = vocab_size
self.embedding = nn.Embedding(vocab_size, d_model)
def forward(self, x):
# (batch, seq_len) --> (batch, seq_len, d_model)
# Multiply by sqrt(d_model) to scale the embeddings according to the paper
return self.embedding(x) * math.sqrt(self.d_model)
class PositionalEncoding(nn.Module):
def __init__(self, d_model: int, seq_len: int, dropout: float) -> None:
super().__init__()
self.d_model = d_model
self.seq_len = seq_len
self.dropout = nn.Dropout(dropout)
# Create a matrix of shape (seq_len, d_model)
pe = torch.zeros(seq_len, d_model)
# Create a vector of shape (seq_len)
position = torch.arange(0, seq_len, dtype=torch.float).unsqueeze(1) # (seq_len, 1)
# Create a vector of shape (d_model)
div_term = torch.exp(torch.arange(0, d_model, 2).float() * (-math.log(10000.0) / d_model)) # (d_model / 2)
# Apply sine to even indices
pe[:, 0::2] = torch.sin(position * div_term) # sin(position * (10000 ** (2i / d_model))
# Apply cosine to odd indices
pe[:, 1::2] = torch.cos(position * div_term) # cos(position * (10000 ** (2i / d_model))
# Add a batch dimension to the positional encoding
pe = pe.unsqueeze(0) # (1, seq_len, d_model)
# Register the positional encoding as a buffer
self.register_buffer('pe', pe)
def forward(self, x):
x = x + (self.pe[:, :x.shape[1], :]).requires_grad_(False) # (batch, seq_len, d_model)
return self.dropout(x)
class ResidualConnection(nn.Module):
def __init__(self, features: int, dropout: float) -> None:
super().__init__()
self.dropout = nn.Dropout(dropout)
self.norm = LayerNormalization(features)
def forward(self, x, sublayer):
return x + self.dropout(sublayer(self.norm(x)))
class MultiHeadAttentionBlock(nn.Module):
def __init__(self, d_model: int, h: int, dropout: float) -> None:
super().__init__()
self.d_model = d_model # Embedding vector size
self.h = h # Number of heads
# Make sure d_model is divisible by h
assert d_model % h == 0, "d_model is not divisible by h"
self.d_k = d_model // h # Dimension of vector seen by each head
self.w_q = nn.Linear(d_model, d_model, bias=False) # Wq
self.w_k = nn.Linear(d_model, d_model, bias=False) # Wk
self.w_v = nn.Linear(d_model, d_model, bias=False) # Wv
self.w_o = nn.Linear(d_model, d_model, bias=False) # Wo
self.dropout = nn.Dropout(dropout)
@staticmethod
def attention(query, key, value, mask, dropout: nn.Dropout):
d_k = query.shape[-1]
# Just apply the formula from the paper
# (batch, h, seq_len, d_k) --> (batch, h, seq_len, seq_len)
attention_scores = (query @ key.transpose(-2, -1)) / math.sqrt(d_k)
if mask is not None:
# Write a very low value (indicating -inf) to the positions where mask == 0
attention_scores.masked_fill_(mask == 0, -1e9)
attention_scores = attention_scores.softmax(dim=-1) # (batch, h, seq_len, seq_len) # Apply softmax
if dropout is not None:
attention_scores = dropout(attention_scores)
# (batch, h, seq_len, seq_len) --> (batch, h, seq_len, d_k)
# return attention scores which can be used for visualization
return (attention_scores @ value), attention_scores
def forward(self, q, k, v, mask):
query = self.w_q(q) # (batch, seq_len, d_model) --> (batch, seq_len, d_model)
key = self.w_k(k) # (batch, seq_len, d_model) --> (batch, seq_len, d_model)
value = self.w_v(v) # (batch, seq_len, d_model) --> (batch, seq_len, d_model)
# (batch, seq_len, d_model) --> (batch, seq_len, h, d_k) --> (batch, h, seq_len, d_k)
query = query.view(query.shape[0], query.shape[1], self.h, self.d_k).transpose(1, 2)
key = key.view(key.shape[0], key.shape[1], self.h, self.d_k).transpose(1, 2)
value = value.view(value.shape[0], value.shape[1], self.h, self.d_k).transpose(1, 2)
# Calculate attention
x, self.attention_scores = MultiHeadAttentionBlock.attention(query, key, value, mask, self.dropout)
# Combine all the heads together
# (batch, h, seq_len, d_k) --> (batch, seq_len, h, d_k) --> (batch, seq_len, d_model)
x = x.transpose(1, 2).contiguous().view(x.shape[0], -1, self.h * self.d_k)
# Multiply by Wo
# (batch, seq_len, d_model) --> (batch, seq_len, d_model)
return self.w_o(x)
class EncoderBlock(nn.Module):
def __init__(self, features: int, self_attention_block: MultiHeadAttentionBlock, feed_forward_block: FeedForwardBlock, dropout: float) -> None:
super().__init__()
self.self_attention_block = self_attention_block
self.feed_forward_block = feed_forward_block
self.residual_connections = nn.ModuleList([ResidualConnection(features, dropout) for _ in range(2)])
def forward(self, x, src_mask):
x = self.residual_connections[0](x, lambda x: self.self_attention_block(x, x, x, src_mask))
x = self.residual_connections[1](x, self.feed_forward_block)
return x
class Encoder(nn.Module):
def __init__(self, features: int, layers: nn.ModuleList) -> None:
super().__init__()
self.layers = layers
self.norm = LayerNormalization(features)
def forward(self, x, mask):
for layer in self.layers:
x = layer(x, mask)
return self.norm(x)
class DecoderBlock(nn.Module):
def __init__(self, features: int, self_attention_block: MultiHeadAttentionBlock, cross_attention_block: MultiHeadAttentionBlock, feed_forward_block: FeedForwardBlock, dropout: float) -> None:
super().__init__()
self.self_attention_block = self_attention_block
self.cross_attention_block = cross_attention_block
self.feed_forward_block = feed_forward_block
self.residual_connections = nn.ModuleList([ResidualConnection(features, dropout) for _ in range(3)])
def forward(self, x, encoder_output, src_mask, tgt_mask):
x = self.residual_connections[0](x, lambda x: self.self_attention_block(x, x, x, tgt_mask))
x = self.residual_connections[1](x, lambda x: self.cross_attention_block(x, encoder_output, encoder_output, src_mask))
x = self.residual_connections[2](x, self.feed_forward_block)
return x
class Decoder(nn.Module):
def __init__(self, features: int, layers: nn.ModuleList) -> None:
super().__init__()
self.layers = layers
self.norm = LayerNormalization(features)
def forward(self, x, encoder_output, src_mask, tgt_mask):
for layer in self.layers:
x = layer(x, encoder_output, src_mask, tgt_mask)
return self.norm(x)
class ProjectionLayer(nn.Module):
def __init__(self, d_model, vocab_size) -> None:
super().__init__()
self.proj = nn.Linear(d_model, vocab_size)
def forward(self, x) -> None:
# (batch, seq_len, d_model) --> (batch, seq_len, vocab_size)
return self.proj(x)
class Transformer(nn.Module):
def __init__(self, encoder: Encoder, decoder: Decoder, src_embed: InputEmbeddings, tgt_embed: InputEmbeddings, src_pos: PositionalEncoding, tgt_pos: PositionalEncoding, projection_layer: ProjectionLayer) -> None:
super().__init__()
self.encoder = encoder
self.decoder = decoder
self.src_embed = src_embed
self.tgt_embed = tgt_embed
self.src_pos = src_pos
self.tgt_pos = tgt_pos
self.projection_layer = projection_layer
def encode(self, src, src_mask):
# (batch, seq_len, d_model)
src = self.src_embed(src)
src = self.src_pos(src)
return self.encoder(src, src_mask)
def decode(self, encoder_output: torch.Tensor, src_mask: torch.Tensor, tgt: torch.Tensor, tgt_mask: torch.Tensor):
# (batch, seq_len, d_model)
tgt = self.tgt_embed(tgt)
tgt = self.tgt_pos(tgt)
return self.decoder(tgt, encoder_output, src_mask, tgt_mask)
def project(self, x):
# (batch, seq_len, vocab_size)
return self.projection_layer(x)
def build_transformer(src_vocab_size: int, tgt_vocab_size: int, src_seq_len: int, tgt_seq_len: int, d_model: int=512, N: int=6, h: int=8, dropout: float=0.1, d_ff: int=2048) -> Transformer:
# Create the embedding layers
src_embed = InputEmbeddings(d_model, src_vocab_size)
tgt_embed = InputEmbeddings(d_model, tgt_vocab_size)
# Create the positional encoding layers
src_pos = PositionalEncoding(d_model, src_seq_len, dropout)
tgt_pos = PositionalEncoding(d_model, tgt_seq_len, dropout)
# Create the encoder blocks
encoder_blocks = []
for _ in range(N):
encoder_self_attention_block = MultiHeadAttentionBlock(d_model, h, dropout)
feed_forward_block = FeedForwardBlock(d_model, d_ff, dropout)
encoder_block = EncoderBlock(d_model, encoder_self_attention_block, feed_forward_block, dropout)
encoder_blocks.append(encoder_block)
# Create the decoder blocks
decoder_blocks = []
for _ in range(N):
decoder_self_attention_block = MultiHeadAttentionBlock(d_model, h, dropout)
decoder_cross_attention_block = MultiHeadAttentionBlock(d_model, h, dropout)
feed_forward_block = FeedForwardBlock(d_model, d_ff, dropout)
decoder_block = DecoderBlock(d_model, decoder_self_attention_block, decoder_cross_attention_block, feed_forward_block, dropout)
decoder_blocks.append(decoder_block)
# Create the encoder and decoder
encoder = Encoder(d_model, nn.ModuleList(encoder_blocks))
decoder = Decoder(d_model, nn.ModuleList(decoder_blocks))
# Create the projection layer
projection_layer = ProjectionLayer(d_model, tgt_vocab_size)
# Create the transformer
transformer = Transformer(encoder, decoder, src_embed, tgt_embed, src_pos, tgt_pos, projection_layer)
# Initialize the parameters
for p in transformer.parameters():
if p.dim() > 1:
nn.init.xavier_uniform_(p)
return transformer |