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