bring back torch.autograd.Function for float8 matmul

Summary:

This is a redo of
#316

With upcoming support of scaling granularities other than tensorwise,
we need a good way to control which gemm kernel to call and how to scale
the input tensors in fwd and bwd. A `torch.autograd.Function` override
is the cleanest way to do that, and in 2024 this now works with
`torch.compile`.

Test Plan:

```
./test/test_everything.sh
```

Reviewers:

Subscribers:

Tasks:

Tags:

ghstack-source-id: 6cb1588bf59be73b5782f6af94e7a360eba7f40e
Pull Request resolved: #336

float8_experimental/float8_linear.py

@@ -71,6 +71,62 @@ def _maybe_initialize_amaxes_scales_for_float8_cast(
         scale.copy_(new_scale)
 
 
+# this code was resurrected from https://github.com/pytorch-labs/float8_experimental/pull/128/files
+@torch._dynamo.allow_in_graph
+class manual_float8_matmul(torch.autograd.Function):
+    """
+    Like torch.matmul, but with the arguments in float8
+    """
+
+    @staticmethod
+    def forward(
+        ctx,
+        input_fp8,
+        weight_fp8_t,
+    ):
+        ctx.save_for_backward(input_fp8, weight_fp8_t)
+        # the reshapes are needed in order to make the shapes compatible with
+        # torch.mm
+        orig_shape = input_fp8.shape
+        input_fp8_reshaped = input_fp8.reshape(-1, orig_shape[-1])
+        res_bits = torch.mm(input_fp8_reshaped, weight_fp8_t)
+        res_bits = res_bits.reshape(*orig_shape[:-1], res_bits.shape[-1])
+        return res_bits
+
+    @staticmethod
+    def backward(ctx, grad_output_fp8):
+        input_fp8, weight_fp8_t = ctx.saved_tensors
+
+        # the reshapes are needed in order to make the shapes compatible with
+        # torch.mm
+        grad_output_fp8_orig_shape = grad_output_fp8.shape
+        grad_output_fp8_reshaped = grad_output_fp8.reshape(
+            -1, grad_output_fp8_orig_shape[-1]
+        )
+
+        # calculate grad_input
+        grad_input = torch.mm(
+            grad_output_fp8_reshaped,
+            weight_fp8_t.t(),
+        )
+        grad_input = grad_input.reshape(
+            *grad_output_fp8_orig_shape[:-1], grad_input.shape[-1]
+        )
+
+        input_fp8_orig_shape = input_fp8.shape
+        input_fp8_reshaped = input_fp8.reshape(-1, input_fp8_orig_shape[-1])
+
+        # calculate grad_weight
+        # Note: the variant below is slightly faster on LLaMa 3 8B pretraining
+        # compared to than calculating `grad_weight_t = input_fp8_t @ grad_output_fp8_reshaped`
+        grad_weight = torch.mm(
+            grad_output_fp8_reshaped.t(),
+            input_fp8_reshaped,
+        )
+
+        return grad_input, grad_weight.t()
+
+
 @torch._dynamo.allow_in_graph
 class NoopFwToFloat8E5M2Bw(torch.autograd.Function):
     """
@@ -393,7 +449,7 @@ def forward(self, input: torch.Tensor) -> torch.Tensor:
         input_fp8 = self.cast_input_to_float8(input, self.is_amax_initialized)
         weight_fp8 = self.cast_weight_to_float8(self.weight, self.is_amax_initialized)
 
-        output = torch.matmul(input_fp8, weight_fp8.t())
+        output = manual_float8_matmul.apply(input_fp8, weight_fp8.t())
 
         # Cast grad_output to float8_e5m2 during backward
         output = self.cast_output_to_float8_in_bw(output)