from minGPT

model.py

@@ -0,0 +1,199 @@
+"""
+GPT model:
+- the initial stem consists of a combination of token encoding and a positional encoding
+- the meat of it is a uniform sequence of Transformer blocks
+    - each Transformer is a sequential combination of a 1-hidden-layer MLP block and a self-attention block
+    - all blocks feed into a central residual pathway similar to resnets
+- the final decoder is a linear projection into a vanilla Softmax classifier
+"""
+
+import math
+import logging
+
+import torch
+import torch.nn as nn
+from torch.nn import functional as F
+
+logger = logging.getLogger(__name__)
+
+class GPTConfig:
+    """ base GPT config, params common to all GPT versions """
+    embd_pdrop = 0.1
+    resid_pdrop = 0.1
+    attn_pdrop = 0.1
+
+    def __init__(self, vocab_size, block_size, **kwargs):
+        self.vocab_size = vocab_size
+        self.block_size = block_size
+        for k,v in kwargs.items():
+            setattr(self, k, v)
+
+class GPT1Config(GPTConfig):
+    """ GPT-1 like network roughly 125M params """
+    n_layer = 12
+    n_head = 12
+    n_embd = 768
+
+class CausalSelfAttention(nn.Module):
+    """
+    A vanilla multi-head masked self-attention layer with a projection at the end.
+    It is possible to use torch.nn.MultiheadAttention here but I am including an
+    explicit implementation here to show that there is nothing too scary here.
+    """
+
+    def __init__(self, config):
+        super().__init__()
+        assert config.n_embd % config.n_head == 0
+        # key, query, value projections for all heads
+        self.key = nn.Linear(config.n_embd, config.n_embd)
+        self.query = nn.Linear(config.n_embd, config.n_embd)
+        self.value = nn.Linear(config.n_embd, config.n_embd)
+        # regularization
+        self.attn_drop = nn.Dropout(config.attn_pdrop)
+        self.resid_drop = nn.Dropout(config.resid_pdrop)
+        # output projection
+        self.proj = nn.Linear(config.n_embd, config.n_embd)
+        # causal mask to ensure that attention is only applied to the left in the input sequence
+        self.register_buffer("mask", torch.tril(torch.ones(config.block_size, config.block_size))
+                                     .view(1, 1, config.block_size, config.block_size))
+        self.n_head = config.n_head
+
+    def forward(self, x):
+        B, T, C = x.size() # batch size, sequence length, embedding dimensionality (n_embd)
+
+        # calculate query, key, values for all heads in batch and move head forward to be the batch dim
+        k = self.key(x).view(B, T, self.n_head, C // self.n_head).transpose(1, 2) # (B, nh, T, hs)
+        q = self.query(x).view(B, T, self.n_head, C // self.n_head).transpose(1, 2) # (B, nh, T, hs)
+        v = self.value(x).view(B, T, self.n_head, C // self.n_head).transpose(1, 2) # (B, nh, T, hs)
+
+        # causal self-attention; Self-attend: (B, nh, T, hs) x (B, nh, hs, T) -> (B, nh, T, T)
+        att = (q @ k.transpose(-2, -1)) * (1.0 / math.sqrt(k.size(-1)))
+        att = att.masked_fill(self.mask[:,:,:T,:T] == 0, float('-inf'))
+        att = F.softmax(att, dim=-1)
+        att = self.attn_drop(att)
+        y = att @ v # (B, nh, T, T) x (B, nh, T, hs) -> (B, nh, T, hs)
+        y = y.transpose(1, 2).contiguous().view(B, T, C) # re-assemble all head outputs side by side
+
+        # output projection
+        y = self.resid_drop(self.proj(y))
+        return y
+
+class Block(nn.Module):
+    """ an unassuming Transformer block """
+
+    def __init__(self, config):
+        super().__init__()
+        self.ln1 = nn.LayerNorm(config.n_embd)
+        self.ln2 = nn.LayerNorm(config.n_embd)
+        self.attn = CausalSelfAttention(config)
+        self.mlp = nn.Sequential(
+            nn.Linear(config.n_embd, 4 * config.n_embd),
+            nn.GELU(),
+            nn.Linear(4 * config.n_embd, config.n_embd),
+            nn.Dropout(config.resid_pdrop),
+        )
+
+    def forward(self, x):
+        x = x + self.attn(self.ln1(x))
+        x = x + self.mlp(self.ln2(x))
+        return x
+
+class GPT(nn.Module):
+    """  the full GPT language model, with a context size of block_size """
+
+    def __init__(self, config):
+        super().__init__()
+
+        # input embedding stem
+        self.tok_emb = nn.Embedding(config.vocab_size, config.n_embd)
+        self.pos_emb = nn.Parameter(torch.zeros(1, config.block_size, config.n_embd))
+        self.drop = nn.Dropout(config.embd_pdrop)
+        # transformer
+        self.blocks = nn.Sequential(*[Block(config) for _ in range(config.n_layer)])
+        # decoder head
+        self.ln_f = nn.LayerNorm(config.n_embd)
+        self.head = nn.Linear(config.n_embd, config.vocab_size, bias=False)
+
+        self.block_size = config.block_size
+        self.apply(self._init_weights)
+
+        logger.info("number of parameters: %e", sum(p.numel() for p in self.parameters()))
+
+    def get_block_size(self):
+        return self.block_size
+
+    def _init_weights(self, module):
+        if isinstance(module, (nn.Linear, nn.Embedding)):
+            torch.nn.init.normal_(module.weight, mean=0.0, std=0.02)
+            if isinstance(module, nn.Linear) and module.bias is not None:
+                torch.nn.init.zeros_(module.bias)
+        elif isinstance(module, nn.LayerNorm):
+            torch.nn.init.zeros_(module.bias)
+            torch.nn.init.ones_(module.weight)
+        elif isinstance(module, GPT):
+            torch.nn.init.normal_(module.pos_emb, mean=0.0, std=0.02)
+
+    def configure_optimizers(self, train_config):
+        """
+        This long function is unfortunately doing something very simple and is being very defensive:
+        We are separating out all parameters of the model into two buckets: those that will experience
+        weight decay for regularization and those that won't (biases, and layernorm/embedding weights).
+        We are then returning the PyTorch optimizer object.
+        """
+
+        # separate out all parameters to those that will and won't experience regularizing weight decay
+        decay = set()
+        no_decay = set()
+        whitelist_weight_modules = (torch.nn.Linear, )
+        blacklist_weight_modules = (torch.nn.LayerNorm, torch.nn.Embedding)
+        for mn, m in self.named_modules():
+            for pn, p in m.named_parameters():
+                fpn = '%s.%s' % (mn, pn) if mn else pn # full param name
+
+                if pn.endswith('bias'):
+                    # all biases will not be decayed
+                    no_decay.add(fpn)
+                elif pn.endswith('weight') and isinstance(m, whitelist_weight_modules):
+                    # weights of whitelist modules will be weight decayed
+                    decay.add(fpn)
+                elif pn.endswith('weight') and isinstance(m, blacklist_weight_modules):
+                    # weights of blacklist modules will NOT be weight decayed
+                    no_decay.add(fpn)
+
+        # special case the position embedding parameter in the root GPT module as not decayed
+        no_decay.add('pos_emb')
+
+        # validate that we considered every parameter
+        param_dict = {pn: p for pn, p in self.named_parameters()}
+        inter_params = decay & no_decay
+        union_params = decay | no_decay
+        assert len(inter_params) == 0, "parameters %s made it into both decay/no_decay sets!" % (str(inter_params), )
+        assert len(param_dict.keys() - union_params) == 0, "parameters %s were not separated into either decay/no_decay set!" \
+                                                    % (str(param_dict.keys() - union_params), )
+
+        # create the pytorch optimizer object
+        optim_groups = [
+            {"params": [param_dict[pn] for pn in sorted(list(decay))], "weight_decay": train_config.weight_decay},
+            {"params": [param_dict[pn] for pn in sorted(list(no_decay))], "weight_decay": 0.0},
+        ]
+        optimizer = torch.optim.AdamW(optim_groups, lr=train_config.learning_rate, betas=train_config.betas)
+        return optimizer
+
+    def forward(self, idx, targets=None):
+        b, t = idx.size()
+        assert t <= self.block_size, "Cannot forward, model block size is exhausted."
+
+        # forward the GPT model
+        token_embeddings = self.tok_emb(idx) # each index maps to a (learnable) vector
+        position_embeddings = self.pos_emb[:, :t, :] # each position maps to a (learnable) vector
+        x = self.drop(token_embeddings + position_embeddings)
+        x = self.blocks(x)
+        x = self.ln_f(x)
+        logits = self.head(x)
+
+        # if we are given some desired targets also calculate the loss
+        loss = None
+        if targets is not None:
+            loss = F.cross_entropy(logits.view(-1, logits.size(-1)), targets.view(-1))
+
+        return logits, loss