[PyTorch] Explicitly specify quantized tensor usages needed for linear op backward (#1646)

Explicitly specify quantized tensor usages needed for linear op backward
Signed-off-by: Tim Moon <tmoon@nvidia.com>

tests/pytorch/test_fusible_ops.py

@@ -1420,15 +1420,17 @@ class TestBasicOps:
             test_device=device,
             test_is_fp8=quantized_compute,
         )
-        if quantized_compute:
-            with torch.no_grad():
-                x_test = x_test.dequantize().requires_grad_()
         dy_ref, dy_test = make_reference_and_test_tensors(
             out_shape,
             test_dtype=dtype,
             test_device=device,
+            test_is_fp8=quantized_compute,
             requires_grad=False,
         )
+        if quantized_compute:
+            with torch.no_grad():
+                x_test = x_test.dequantize().requires_grad_()
+                dy_test = dy_test.dequantize()
 
         # Plain PyTorch implementation
         y_ref: torch.Tensor
@@ -1459,6 +1461,7 @@ class TestBasicOps:
             swiglu=te_ops.SwiGLU,
         )[activation]
         forward = te_ops.Sequential(
+            te_ops.Quantize(forward=False, backward=quantized_compute),
             make_op(),
             te_ops.Quantize(forward=quantized_compute, backward=False),
         )

transformer_engine/pytorch/ops/basic/basic_linear.py

@@ -523,7 +523,7 @@ class BasicLinear(BasicOperation):
 
         # Configure input tensor for backward pass
         if own_quantized_x_local:
-            x_local.update_usage(rowwise_usage=False)
+            x_local.update_usage(rowwise_usage=False, columnwise_usage=True)
 
         # Detach input tensor if needed
         # Note: PyTorch autograd produces esoteric errors if we save
@@ -679,7 +679,9 @@ class BasicLinear(BasicOperation):
                         quantizer=input_quantizer,
                     )
                 else:
-                    if not isinstance(x_local, QuantizedTensor):
+                    if isinstance(x_local, QuantizedTensor):
+                        x_local.update_usage(columnwise_usage=True)
+                    else:
                         x_local = input_quantizer(x_local)
                     x = x_local
             else:
@@ -706,14 +708,18 @@ class BasicLinear(BasicOperation):
                 raise ValueError("Weight tensor is required to compute input grad")
             w = weight
             w_is_quantized = isinstance(w, QuantizedTensor)
-            if with_quantized_compute and not w_is_quantized:
+            if with_quantized_compute:
+                if w_is_quantized:
+                    w.update_usage(columnwise_usage=True)
+                else:
                     if weight_quantizer is None:
                         raise ValueError("Missing quantizer for weight tensor")
                     weight_quantizer.set_usage(columnwise=True)
                     w = weight_quantizer(w)
-            elif not with_quantized_compute and w_is_quantized:
-                w = w.dequantize()
-            if not with_quantized_compute and w.dtype != dtype:
+            else:
+                if w_is_quantized:
+                    w = w.dequantize(dtype=dtype)
+                elif w.dtype != dtype:
                     w = w.to(dtype=dtype)
 
             # Synchronize tensor-parallel communication
@@ -867,8 +873,8 @@ class BasicLinear(BasicOperation):
             # Configure quantizers
             # Note: We cache the quantized input for backward pass,
             # but discard the quantized weights.
-            input_quantizer.set_usage(columnwise=weight_requires_grad)
-            weight_quantizer.set_usage(columnwise=False)
+            input_quantizer.set_usage(rowwise=True, columnwise=weight_requires_grad)
+            weight_quantizer.set_usage(rowwise=True, columnwise=False)
 
         # Get autocast dtype if needed
         dtype = None