mx: remove differentiable casts

Summary:

This functionality is not used right now, removing so we can move
faster. We can add it back in the future if needed.

Test Plan:

```
pytest test/prototype/mx_formats/ -s -x
```

Reviewers:

Subscribers:

Tasks:

Tags:

ghstack-source-id: ea12f77
ghstack-comment-id: 2761693552
Pull Request resolved: #1978

Author: vkuzo

test/prototype/mx_formats/test_mx_tensor.py

@@ -443,18 +443,6 @@ def test_transpose(elem_dtype, fp4_triton):
     torch.testing.assert_close(tensor_mx_dq_t, tensor_mx_t_dq, atol=0, rtol=0)
 
 
-@pytest.mark.skipif(not torch.cuda.is_available(), reason="CUDA not available")
-@pytest.mark.parametrize("elem_dtype", SUPPORTED_ELEM_DTYPES)
-def test_cast_autograd(elem_dtype):
-    x = torch.arange(8, device="cuda").bfloat16().requires_grad_()
-    grad = torch.arange(8, device="cuda").bfloat16() * 0.5
-    block_size = 8
-    x_mx = MXTensor.to_mx(x, elem_dtype, block_size)
-    x_dq = x_mx.to_dtype(torch.bfloat16)
-    x_dq.backward(gradient=grad)
-    torch.testing.assert_close(grad, x.grad, atol=0, rtol=0)
-
-
 @pytest.mark.skipif(not torch.cuda.is_available(), reason="CUDA not available")
 @pytest.mark.parametrize("elem_dtype", SUPPORTED_ELEM_DTYPES)
 def test_view(elem_dtype):

torchao/prototype/mx_formats/mx_tensor.py

@@ -467,65 +467,6 @@ def tensor_size_fp6x4_to_hpx3(orig_size, is_contiguous):
     return new_size
 
 
-@torch._dynamo.allow_in_graph
-class ToMXConstrFunc(torch.autograd.Function):
-    """
-    Differentiable cast to MX, no-op in backward
-    """
-
-    @staticmethod
-    def forward(
-        ctx,
-        data_hp,
-        elem_dtype,
-        block_size,
-        scaling_mode,
-        use_fp4_custom_triton_dequant_kernel,
-        gemm_kernel_choice,
-        pack_fp6,
-    ):
-        scale_e8m0_biased, data_lp = to_mx(
-            data_hp, elem_dtype, block_size, scaling_mode, pack_fp6=pack_fp6
-        )
-        return MXTensor(
-            scale_e8m0_biased,
-            data_lp,
-            elem_dtype,
-            block_size,
-            data_hp.dtype,
-            use_fp4_custom_triton_dequant_kernel,
-            gemm_kernel_choice,
-            pack_fp6,
-        )
-
-    @staticmethod
-    def backward(ctx, g):
-        return g, None, None, None, None, None, None
-
-
-@torch._dynamo.allow_in_graph
-class FromMXConstrFunc(torch.autograd.Function):
-    """
-    Differentiable cast from MX, no-op in backward
-    """
-
-    @staticmethod
-    def forward(ctx, tensor_lp, target_dtype):
-        return to_dtype(
-            tensor_lp._data,
-            tensor_lp._scale_e8m0,
-            tensor_lp._elem_dtype,
-            tensor_lp._block_size,
-            target_dtype,
-            tensor_lp._use_fp4_custom_triton_dequant_kernel,
-            tensor_lp._pack_fp6,
-        )
-
-    @staticmethod
-    def backward(ctx, g):
-        return g, None, None
-
-
 class MXTensor(torch.Tensor):
     def __new__(
         cls,
@@ -627,7 +568,15 @@ def __torch_dispatch__(cls, func, types, args, kwargs=None):
         raise NotImplementedError(f"{func} not implemented")
 
     def to_dtype(self, target_dtype):
-        return FromMXConstrFunc.apply(self, target_dtype)
+        return to_dtype(
+            self._data,
+            self._scale_e8m0,
+            self._elem_dtype,
+            self._block_size,
+            target_dtype,
+            self._use_fp4_custom_triton_dequant_kernel,
+            self._pack_fp6,
+        )
 
     @staticmethod
     @torch._dynamo.allow_in_graph
@@ -640,11 +589,15 @@ def to_mx(
         gemm_kernel_choice: MXGemmKernelChoice = MXGemmKernelChoice.EMULATED,
         pack_fp6: bool = False,
     ):
-        return ToMXConstrFunc.apply(
-            data_hp,
+        scale_e8m0_biased, data_lp = to_mx(
+            data_hp, elem_dtype, block_size, scaling_mode, pack_fp6
+        )
+        return MXTensor(
+            scale_e8m0_biased,
+            data_lp,
             elem_dtype,
             block_size,
-            scaling_mode,
+            data_hp.dtype,
             use_fp4_custom_triton_dequant_kernel,
             gemm_kernel_choice,
             pack_fp6,