lyceum-env/classes/english/transformer.py

import torch
import torch.nn as nn

class SelfAttention(nn.Module):
    def __init__(self, embed_size, heads):
        super(SelfAttention, self).__init__()

        self.embed_size = embed_size
        self.heads = heads
        self.head_dim = embed_size // heads

        assert (self.head_dim * heads == embed_size), "Embed size needs to be divisible by heads"

        self.values = nn.Linear(self.head_dim, self.head_dim, bias=False)
        self.keys = nn.Linear(self.head_dim, self.head_dim, bias=False)
        self.queries = nn.Linear(self.head_dim, self.head_dim, bias=False)
        self.fc_out = nn.Linear(heads*self.head_dim, self.embed_size)

    def forward(self, values, keys, query, mask):
        N = query.shape[0]
        value_len, key_len, query_len = values.shape[1], keys.shape[1], query.shape[1]

        # Split embedding into self.heads pieces
        values = values.reshape(N, value_len, self.heads, self.head_dim)
        keys = keys.reshape(N, key_len, self.heads, self.head_dim)
        query = query.reshape(N, query_len, self.heads, self.head_dim)

        values = self.values(values)
        keys = self.keys(keys)
        query = self.queries(query)
        
        # einsum is black magic i guess
        energy = torch.einsum("nqhd,nkhd->nhqk", [query, keys])

        if mask is not None:
            energy = energy.masked_fill(mask==0, float("-1e20"))

        attention = torch.softmax(energy / (self.embed_size**(1/2)), dim=3)

        # once again, we say the magic words, and then we flatten the einsum
        out = torch.einsum("nhql,nlhd->nqhd", [attention, values]).reshape(
            N, query_len, self.heads*self.head_dim
        )

        out = self.fc_out(out)
        return out

class TransformerBlock(nn.Module):
    def __init__(self, embed_size, heads, dropout, forward_expansion):
        super(TransformerBlock, self).__init__()
        self.attention = SelfAttention(embed_size, heads)

        self.norm1 = nn.LayerNorm(embed_size)
        self.norm2 = nn.LayerNorm(embed_size)

        self.feed_forward = nn.Sequential(
            nn.Linear(embed_size, forward_expansion*embed_size),
            nn.ReLU(),
            nn.Linear(forward_expansion*embed_size, embed_size)
        )

        self.dropout = nn.Dropout(dropout)

    def forward(self, value, key, query, mask):
        attention = self.attention(value, key, query, mask)

        x = self.dropout(self.norm1(attention+query))
        forward = self.feed_forward(x)
        out = self.dropout(self.norm2(forward + x))
        return out

class Encoder(nn.Module):
    def __init__(self, src_vocab_size, embed_size, num_layers, heads, device, forward_expansion, dropout, max_length):
        super(Encoder, self).__init__()
        self.embed_size = embed_size
        self.device = device
        self.word_embedding = nn.Embedding(src_vocab_size, embed_size)
        self.position_embedding = nn.Embedding(max_length, embed_size)

        self.layers = nn.ModuleList(
            [
                TransformerBlock(
                    embed_size,
                    heads,
                    dropout=dropout,
                    forward_expansion=forward_expansion
                )
                for _ in range(num_layers)
            ]
        )
        self.dropout = nn.Dropout(dropout)

    def forward(self, x, mask):
        N, seq_len = x.shape
        positions = torch.arange(0, seq_len).expand(N, seq_len).to(self.device)

        out = self.dropout(self.word_embedding(x)+self.position_embedding(positions))

        for layer in self.layers:
            out = layer(out, out, out, mask)

        return out

class DecoderBlock(nn.Module):
    def __init__(self, embed_size, heads, forward_expansion, dropout, device):
        super(DecoderBlock, self).__init__()
        self.attention = SelfAttention(embed_size, heads)
        self.norm = nn.LayerNorm(embed_size)
        self.transformer_block = TransformerBlock(
            embed_size, heads, dropout, forward_expansion
        )
        self.dropout = nn.Dropout(dropout)

    def forward(self, x, value, key, src_mask, trg_mask):
        attention = self.attention(x, x, x, trg_mask)
        query = self.dropout(self.norm(attention+x))
        out = self.transformer_block(value, key, query, src_mask)
        return out

class Decoder(nn.Module):
    def __init__(self, trg_vocab_size, embed_size, num_layers, heads, forward_expansion, dropout, device, max_length):
        super(Decoder, self).__init__()
        self.device = device
        self.word_embedding = nn.Embedding(trg_vocab_size, embed_size)
        self.position_embedding = nn.Embedding(max_length, embed_size)

        self.layers = nn.ModuleList(
            [
                DecoderBlock(
                    embed_size,
                    heads,
                    forward_expansion,
                    dropout,
                    device
                )
                for _ in range(num_layers)
            ]
        )

        self.fc_out = nn.Linear(embed_size, trg_vocab_size)
        self.dropout = nn.Dropout(dropout)

    def forward(self, x, enc_out, src_mask, trg_mask):
        N, seq_len = x.shape
        positions = torch.arange(0, seq_len).expand(N, seq_len).to(self.device)
        x = self.dropout(self.word_embedding(x)+self.position_embedding(positions))

        for layer in self.layers:
            x = layer(x, enc_out, enc_out, src_mask, trg_mask)

        out = self.fc_out(x)
        return out

class Transformer(nn.Module):
    def __init__(self, src_vocab_size, trg_vocab_size, src_pad_idx, trg_pad_idx, embed_size=256, num_layers=6, forward_expansion=4, num_heads=8, dropout=0, device="cuda", max_length=50000):
        super(Transformer, self).__init__()
        self.encoder = Encoder(
            src_vocab_size,
            embed_size,
            num_layers,
            num_heads,
            device,
            forward_expansion,
            dropout,
            max_length
        )

        self.decoder = Decoder(
            trg_vocab_size,
            embed_size,
            num_layers,
            num_heads,
            forward_expansion,
            dropout,
            device,
            max_length
        )

        self.src_pad_idx = src_pad_idx
        self.trg_pad_idx = trg_pad_idx
        self.device = device

    def make_src_mask(self, src):
        src_mask = (src != self.src_pad_idx).unsqueeze(1).unsqueeze(2)
        return src_mask.to(self.device)

    def make_trg_mask(self, trg):
        N, trg_len = trg.shape
        trg_mask = torch.tril(torch.ones(trg_len, trg_len)).expand(
            N, 1, trg_len, trg_len
        )
        return trg_mask.to(self.device)

    def forward(self, src, trg):
        src_mask = self.make_src_mask(src)
        trg_mask = self.make_trg_mask(trg)

        enc_src = self.encoder(src, src_mask)
        out = self.decoder(trg, enc_src, src_mask, trg_mask)
        return out
    

        
if __name__ == "__main__":
    device = torch.device("cuda" if torch.cuda.is_available() else "cpu")

    x = torch.tensor([[1, 5, 6, 4, 3, 9, 5, 2, 0], [1, 8, 7, 3, 4, 5, 6, 7 ,2]]).to(device)
    trg = torch.tensor([[1, 7, 4, 3, 5, 9, 2, 0], [1, 5, 6, 2, 4, 7, 6, 2]]).to(device)

    src_pad_idx = 0
    trg_pad_idx = 0
    src_vocab_size = 10
    trg_vocab_size = 10
    model = Transformer(src_vocab_size, trg_vocab_size, src_pad_idx, trg_pad_idx, device).to(device)
    out = model(x, trg[:, :-1])
    print(out.shape)