wire up fused qkv and glu toggles

nanochat/gpt.py

@@ -4,16 +4,18 @@ Notable features:
 - rotary embeddings (and no positional embeddings)
 - QK norm
 - untied weights for token embedding and lm_head
-- relu^2 activation in MLP
+- relu^2 / gated MLPs with width scaling
 - norm after token embedding
 - no learnable params in rmsnorm
 - no bias in linear layers
 - Multi-Query Attention (MQA) support for more efficient inference
+- Optional fused QKV projection for fewer matmuls
 """
 
 import math
 from functools import partial
 from dataclasses import dataclass
+from typing import Optional
 
 import torch
 import torch.nn as nn
@@ -31,6 +33,10 @@ class GPTConfig:
     n_head: int = 6 # number of query heads
     n_kv_head: int = 6 # number of key/value heads (MQA)
     n_embd: int = 768
+    use_fused_qkv: bool = True
+    mlp_type: str = "relu2" # choices: relu2, swiglu, geglu
+    mlp_width_mult: float = 4.0
+    mlp_glu_width_mult: Optional[float] = None
 
 
 def norm(x):
@@ -69,8 +75,14 @@ class CausalSelfAttention(nn.Module):
         self.n_kv_head = config.n_kv_head
         self.n_embd = config.n_embd
         self.head_dim = self.n_embd // self.n_head
+        self.total_qkv_heads = self.n_head + 2 * self.n_kv_head
         assert self.n_embd % self.n_head == 0
         assert self.n_kv_head <= self.n_head and self.n_head % self.n_kv_head == 0
+        self.use_fused_qkv = config.use_fused_qkv
+        if self.use_fused_qkv:
+            # Single projection keeps matmul count minimal; split happens in forward pass.
+            self.c_attn = nn.Linear(self.n_embd, self.total_qkv_heads * self.head_dim, bias=False)
+        else:
             self.c_q = nn.Linear(self.n_embd, self.n_head * self.head_dim, bias=False)
             self.c_k = nn.Linear(self.n_embd, self.n_kv_head * self.head_dim, bias=False)
             self.c_v = nn.Linear(self.n_embd, self.n_kv_head * self.head_dim, bias=False)
@@ -80,6 +92,11 @@ class CausalSelfAttention(nn.Module):
         B, T, C = x.size()
 
         # Project the input to get queries, keys, and values
+        if self.use_fused_qkv:
+            qkv = self.c_attn(x)
+            qkv = qkv.view(B, T, self.total_qkv_heads, self.head_dim)
+            q, k, v = torch.split(qkv, [self.n_head, self.n_kv_head, self.n_kv_head], dim=2)
+        else:
             q = self.c_q(x).view(B, T, self.n_head, self.head_dim)
             k = self.c_k(x).view(B, T, self.n_kv_head, self.head_dim)
             v = self.c_v(x).view(B, T, self.n_kv_head, self.head_dim)
@@ -125,15 +142,62 @@ class CausalSelfAttention(nn.Module):
         y = self.c_proj(y)
         return y
 
+    def _load_from_state_dict(self, state_dict, prefix, local_metadata, strict,
+                               missing_keys, unexpected_keys, error_msgs):
+        fused_key = prefix + "c_attn.weight"
+        q_key = prefix + "c_q.weight"
+        k_key = prefix + "c_k.weight"
+        v_key = prefix + "c_v.weight"
+        has_fused = fused_key in state_dict
+        has_split = all(key in state_dict for key in (q_key, k_key, v_key))
+
+        if self.use_fused_qkv and not has_fused and has_split:
+            # Convert legacy split weights into the fused layout expected by this module.
+            q = state_dict.pop(q_key)
+            k = state_dict.pop(k_key)
+            v = state_dict.pop(v_key)
+            state_dict[fused_key] = torch.cat([q, k, v], dim=0)
+        elif not self.use_fused_qkv and has_fused and not has_split:
+            fused = state_dict.pop(fused_key)
+            q_out = self.n_head * self.head_dim
+            kv_out = self.n_kv_head * self.head_dim
+            state_dict[q_key] = fused[:q_out]
+            state_dict[k_key] = fused[q_out:q_out + kv_out]
+            state_dict[v_key] = fused[q_out + kv_out:]
+
+        super()._load_from_state_dict(state_dict, prefix, local_metadata, strict,
+                                       missing_keys, unexpected_keys, error_msgs)
+
 
 class MLP(nn.Module):
     def __init__(self, config):
         super().__init__()
-        self.c_fc = nn.Linear(config.n_embd, 4 * config.n_embd, bias=False)
+        self.mlp_type = config.mlp_type.lower()
-        self.c_proj = nn.Linear(4 * config.n_embd, config.n_embd, bias=False)
+        base_hidden = max(1, int(config.mlp_width_mult * config.n_embd))
+        if self.mlp_type in {"swiglu", "geglu"}:
+            width_mult = config.mlp_glu_width_mult
+            if width_mult is None:
+                # 2/3 factor keeps parameter count aligned with the ReLU^2 baseline.
+                width_mult = config.mlp_width_mult * (2.0 / 3.0)
+            hidden_dim = max(1, int(width_mult * config.n_embd))
+            self.c_fc = nn.Linear(config.n_embd, 2 * hidden_dim, bias=False)
+            self.c_proj = nn.Linear(hidden_dim, config.n_embd, bias=False)
+            self.hidden_dim = hidden_dim
+        else:
+            hidden_dim = base_hidden
+            self.c_fc = nn.Linear(config.n_embd, hidden_dim, bias=False)
+            self.c_proj = nn.Linear(hidden_dim, config.n_embd, bias=False)
+            self.hidden_dim = hidden_dim
 
     def forward(self, x):
         x = self.c_fc(x)
+        if self.mlp_type == "swiglu":
+            x1, x2 = torch.chunk(x, 2, dim=-1)
+            x = F.silu(x1) * x2
+        elif self.mlp_type == "geglu":
+            x1, x2 = torch.chunk(x, 2, dim=-1)
+            x = F.gelu(x1, approximate="tanh") * x2
+        else:
             x = F.relu(x).square()
         x = self.c_proj(x)
         return x
@@ -169,7 +233,8 @@ class GPT(nn.Module):
         cos, sin = self._precompute_rotary_embeddings(self.rotary_seq_len, head_dim)
         self.register_buffer("cos", cos, persistent=False) # persistent=False means it's not saved to the checkpoint
         self.register_buffer("sin", sin, persistent=False)
-        # Cast the embeddings from fp32 to bf16: optim can tolerate it and it saves memory: both in the model and the activations
+        # Cast embeddings to bf16 only when CUDA is available to keep CPU inference/tests simple.
+        if torch.cuda.is_available():
             self.transformer.wte.to(dtype=torch.bfloat16)
 
     def init_weights(self):

tests/test_attention_fusion.py

@@ -0,0 +1,59 @@
+import torch
+from dataclasses import replace
+
+from nanochat.gpt import GPT, GPTConfig
+
+
+def _tiny_config(**kwargs):
+    base = GPTConfig(
+        sequence_len=16,
+        vocab_size=128,
+        n_layer=1,
+        n_head=4,
+        n_kv_head=2,
+        n_embd=64,
+        use_fused_qkv=False,
+        mlp_type="relu2",
+    )
+    return replace(base, **kwargs)
+
+
+def test_fused_qkv_matches_legacy_split_projection():
+    torch.manual_seed(0)
+    split_cfg = _tiny_config(use_fused_qkv=False)
+    fused_cfg = replace(split_cfg, use_fused_qkv=True)
+
+    split_model = GPT(split_cfg)
+    split_model.init_weights()
+    state = split_model.state_dict()
+
+    fused_model = GPT(fused_cfg)
+    fused_model.init_weights()
+    fused_model.load_state_dict(state, strict=True)
+
+    tokens = torch.randint(0, split_cfg.vocab_size, (2, 5))
+    with torch.no_grad():
+        logits_split = split_model(tokens)
+        logits_fused = fused_model(tokens)
+
+    assert torch.allclose(logits_split, logits_fused, atol=1e-5)
+
+
+def test_split_loads_from_fused_state_dict():
+    torch.manual_seed(1)
+    fused_cfg = _tiny_config(use_fused_qkv=True)
+    fused_model = GPT(fused_cfg)
+    fused_model.init_weights()
+    state = fused_model.state_dict()
+
+    split_cfg = replace(fused_cfg, use_fused_qkv=False)
+    split_model = GPT(split_cfg)
+    split_model.init_weights()
+    split_model.load_state_dict(state, strict=True)
+
+    tokens = torch.randint(0, split_cfg.vocab_size, (1, 7))
+    with torch.no_grad():
+        logits_split = split_model(tokens)
+        logits_fused = fused_model(tokens)
+
+    assert torch.allclose(logits_split, logits_fused, atol=1e-5)

tests/test_mlp_variants.py

@@ -0,0 +1,48 @@
+import torch
+from dataclasses import replace
+
+from nanochat.gpt import GPTConfig, MLP
+
+
+def _mlp_config(**kwargs):
+    cfg = GPTConfig(
+        n_embd=32,
+        mlp_type="relu2",
+        mlp_width_mult=4.0,
+        mlp_glu_width_mult=None,
+        n_layer=1,
+        n_head=4,
+        n_kv_head=4,
+        sequence_len=8,
+        vocab_size=128,
+    )
+    return replace(cfg, **kwargs)
+
+
+def test_relu2_mlp_shape():
+    cfg = _mlp_config(mlp_type="relu2")
+    mlp = MLP(cfg)
+    assert mlp.c_fc.weight.shape[0] == int(cfg.mlp_width_mult * cfg.n_embd)
+    x = torch.randn(2, 5, cfg.n_embd)
+    out = mlp(x)
+    assert out.shape == x.shape
+
+
+def test_swiglu_width_scaling_defaults_to_two_thirds():
+    cfg = _mlp_config(mlp_type="swiglu")
+    mlp = MLP(cfg)
+    expected_hidden = int(cfg.n_embd * cfg.mlp_width_mult * (2.0 / 3.0))
+    assert mlp.c_fc.weight.shape[0] == 2 * expected_hidden
+    x = torch.randn(4, 3, cfg.n_embd)
+    out = mlp(x)
+    assert out.shape == x.shape
+
+
+def test_geglu_respects_custom_width():
+    cfg = _mlp_config(mlp_type="geglu", mlp_glu_width_mult=0.5)
+    mlp = MLP(cfg)
+    expected_hidden = int(cfg.n_embd * 0.5)
+    assert mlp.c_fc.weight.shape[0] == 2 * expected_hidden
+    x = torch.randn(3, 2, cfg.n_embd)
+    out = mlp(x)
+    assert out.shape == x.shape