Merge branch 'Chetter2-patch-1'

nanochat/fp8.py

@@ -123,19 +123,16 @@ def _to_col_major(x):
 class _Float8Matmul(torch.autograd.Function):
     """Custom autograd for the three FP8 GEMMs of a Linear layer.
 
-    The forward saves input and weight in their original precision for the
+    The forward quantizes input and weight to FP8 and saves 
-    backward pass. Each GEMM independently re-quantizes its operands to FP8.
+    the quantized tensors + scales for backward.
-    (We don't reuse the forward's FP8 tensors in backward — the backward might
-    want different precision, and saving FP8 would lose information.)
     """
 
     @staticmethod
     def forward(ctx, input_2d, weight):
-        ctx.save_for_backward(input_2d, weight)
-
         # Quantize both operands to e4m3 (higher precision format)
         input_fp8, input_inv = _to_fp8(input_2d, torch.float8_e4m3fn)
         weight_fp8, weight_inv = _to_fp8(weight, torch.float8_e4m3fn)
+        ctx.save_for_backward(input_fp8, input_inv, weight_fp8, weight_inv)
 
         # output = input @ weight.T
         # input_fp8 is [B, K] contiguous = row-major (good for first arg)
@@ -156,13 +153,12 @@ class _Float8Matmul(torch.autograd.Function):
 
     @staticmethod
     def backward(ctx, grad_output):
-        input_2d, weight = ctx.saved_tensors
+        in_fp8, in_inv, w_fp8, w_inv = ctx.saved_tensors
 
         # === GEMM 1: grad_input = grad_output @ weight ===
         # Shapes: [B, N] @ [N, K] -> [B, K]
         # Gradients use e5m2 (wider range), weights use e4m3 (higher precision)
         go_fp8, go_inv = _to_fp8(grad_output, torch.float8_e5m2)
-        w_fp8, w_inv = _to_fp8(weight, torch.float8_e4m3fn)
         # go_fp8 is [B, N] contiguous = row-major, good for first arg
         # w_fp8 is [N, K] contiguous = row-major, need column-major for second arg
         w_col = _to_col_major(w_fp8)
@@ -177,17 +173,15 @@ class _Float8Matmul(torch.autograd.Function):
 
         # === GEMM 2: grad_weight = grad_output.T @ input ===
         # Shapes: [N, B] @ [B, K] -> [N, K]
-        go_fp8_2, go_inv_2 = _to_fp8(grad_output, torch.float8_e5m2)
+        # go_fp8 is [B, N] contiguous, we need go.T = [N, B] as first arg.
-        in_fp8, in_inv = _to_fp8(input_2d, torch.float8_e4m3fn)
-        # go_fp8_2 is [B, N] contiguous, we need go.T = [N, B] as first arg.
         # Transposing gives column-major, but first arg needs row-major,
         # so we must call .contiguous() to physically rearrange the memory.
-        go_T = go_fp8_2.t().contiguous()  # [N, B] row-major
+        go_T = go_fp8.t().contiguous()  # [N, B] row-major
         in_col = _to_col_major(in_fp8)    # [B, K] column-major
         grad_weight = torch._scaled_mm(
             go_T,
             in_col,
-            scale_a=go_inv_2,
+            scale_a=go_inv,
             scale_b=in_inv,
             out_dtype=grad_output.dtype,
             use_fast_accum=False,

scripts/base_train.py

@@ -170,20 +170,22 @@ if args.fp8:
         # from torchao.float8 import Float8LinearConfig, convert_to_float8_training
         import torch.nn as nn
 
-        # Filter: only convert layers with dimensions divisible by 16 (FP8 hardware requirement)
+        # Filter: dims must be divisible by 16 (FP8 hardware requirement) large enough
         def fp8_module_filter(mod: nn.Module, fqn: str) -> bool:
             if not isinstance(mod, nn.Linear):
                 return False
-            # FP8 requires both in_features and out_features divisible by 16
             if mod.in_features % 16 != 0 or mod.out_features % 16 != 0:
                 return False
+            if min(mod.in_features, mod.out_features) < 128:
+                return False
             return True
 
         fp8_config = Float8LinearConfig.from_recipe_name(args.fp8_recipe)
+        num_linear = sum(1 for m in model.modules() if isinstance(m, nn.Linear))
         convert_to_float8_training(model, config=fp8_config, module_filter_fn=fp8_module_filter)
-        num_fp8_layers = sum(1 for m in model.modules() if 'Float8' in type(m).__name__)
+        num_fp8 = sum(1 for m in model.modules() if 'Float8' in type(m).__name__)
-        num_skipped = sum(1 for m in model.modules() if isinstance(m, nn.Linear)) - num_fp8_layers
+        num_skipped = num_linear - num_fp8
-        print0(f"✓ FP8 training enabled ({args.fp8_recipe} scaling) - converted {num_fp8_layers} layers, skipped {num_skipped} (dims not divisible by 16)")
+        print0(f"✓ FP8 training enabled ({args.fp8_recipe} scaling) - converted {num_fp8}/{num_linear} linear layers, skipped {num_skipped} (too small)")
 
 # Context manager to temporarily disable FP8 so that model evaluation remains in BF16
 @contextmanager