Fix NVFP4 QAT backward typo (#3478)

**Summary:** Fix `to_dtype` -> `dequantize`. This was broken in
#3169.

**Test Plan:**
```
python test/quantization/test_qat.py -k nvfp4_training
```

Author: andrewor14

test/quantization/test_qat.py

@@ -2149,6 +2149,40 @@ def test_qat_nvfp4(self, use_per_tensor_scale: bool):
         sqnr = compute_error(out, baseline_out).item()
         self.assertGreaterEqual(sqnr, float("inf"))
 
+    @unittest.skipIf(not is_sm_at_least_89(), "Need sm89+")
+    @unittest.skipIf(not _CUDA_IS_AVAILABLE, "skipping when cuda is not available")
+    @parametrize("use_per_tensor_scale", [True, False])
+    def test_qat_nvfp4_training(self, use_per_tensor_scale: bool):
+        from torchao.prototype.mx_formats import NVFP4DynamicActivationNVFP4WeightConfig
+
+        torch.manual_seed(self.SEED)
+        m = M().cuda()
+        base_config = NVFP4DynamicActivationNVFP4WeightConfig(
+            use_dynamic_per_tensor_scale=use_per_tensor_scale
+        )
+        quantize_(m, QATConfig(base_config, step="prepare"))
+
+        # Simulate training
+        num_steps = 10
+        optimizer = torch.optim.SGD(
+            m.parameters(), lr=0.001, momentum=0.9, weight_decay=1e-5
+        )
+        loss_fn = torch.nn.CrossEntropyLoss()
+        for i in range(num_steps):
+            example_inputs = m.example_inputs("cuda")
+            prev_weight = copy.deepcopy(m.linear1.weight)
+            optimizer.zero_grad()
+            target = torch.randn(1, 512).float().cuda()
+            out = m(*example_inputs)
+            loss = loss_fn(out, target)
+            loss.backward()
+            optimizer.step()
+            # Assert that weights have valid gradients and are being updated
+            new_weight = m.linear1.weight
+            self.assertIsNotNone(new_weight.grad)
+            self.assertNotEqual(torch.count_nonzero(new_weight.grad), 0)
+            self.assertFalse(torch.equal(new_weight, prev_weight))
+
     @unittest.skipIf(not _CUDA_IS_AVAILABLE, "skipping when cuda is not available")
     @unittest.skipIf(
         not _is_fbgemm_gpu_genai_available(), "Requires fbgemm-gpu-genai >= 1.2.0"

torchao/prototype/qat/nvfp4.py

@@ -91,8 +91,8 @@ def backward(ctx, grad_output: torch.Tensor) -> torch.Tensor:
         _input, weight = ctx.saved_tensors
         assert isinstance(_input, NVFP4Tensor)
         assert isinstance(weight, NVFP4Tensor)
-        _input = _input.to_dtype(_input._orig_dtype)
-        weight = weight.to_dtype(weight._orig_dtype)
+        _input = _input.dequantize(_input._orig_dtype)
+        weight = weight.dequantize(weight._orig_dtype)
         grad_input = torch.mm(grad_output, weight)
         grad_weight = torch.mm(grad_output.t(), _input)
         return grad_input, grad_weight, None, None, None