[Relax][Torch] Fix from_exported_program crash with FakeTensor and lifted tensors (#18407)

Author: Dayuxiaoshui

python/tvm/relax/frontend/torch/exported_program_translator.py

@@ -49,17 +49,18 @@ def _convert_pytorch_tensor_to_tvm(tensor_value: torch.Tensor) -> tvm.runtime.Te
             The converted TVM tensor.
         """
         # Fix for Issue #18407: Handle FakeTensor and lifted tensors (from torch.export)
-        # Check if this is a FakeTensor or tensor subclass that doesn't support .numpy()
+        # FakeTensor is an internal PyTorch API that may not be available in all versions
         try:
-            # Check if it's a FakeTensor
-            if hasattr(torch, '_subclasses') and hasattr(torch._subclasses, 'fake_tensor'):
-                if isinstance(tensor_value, torch._subclasses.fake_tensor.FakeTensor):
-                    # Create a real tensor with the same shape and dtype
-                    real_tensor = torch.zeros(tensor_value.shape, dtype=tensor_value.dtype)
-                    return tvm.runtime.tensor(real_tensor.numpy())
+            # Check if it's a FakeTensor from torch._subclasses, which is an internal API
+            FakeTensor = torch._subclasses.fake_tensor.FakeTensor
+            if isinstance(tensor_value, FakeTensor):
+                # Create a real tensor with the same shape and dtype as a placeholder
+                real_tensor = torch.zeros(tensor_value.shape, dtype=tensor_value.dtype)
+                return tvm.runtime.tensor(real_tensor.numpy())
         except (AttributeError, ImportError):
+            # FakeTensor class might not exist in this torch version, or other import issue
             pass
-        
+
         # PyTorch sparse tensors (layout != torch.strided) must be converted to dense.
         if tensor_value.layout != torch.strided:
             tensor_to_convert = tensor_value.to_dense()
@@ -78,8 +79,12 @@ def _convert_pytorch_tensor_to_tvm(tensor_value: torch.Tensor) -> tvm.runtime.Te
                 return tvm.runtime.tensor(tensor_cpu.numpy())
             except RuntimeError as e:
                 # Fix for Issue #18407: Handle tensor subclasses that don't support .numpy()
-                # This can happen with lifted tensors from torch.export
-                if "tensor subclasses" in str(e) or "FakeTensor" in str(e):
+                # This can happen with lifted tensors from torch.export that slip through
+                # the FakeTensor check above (e.g., other tensor subclasses)
+                # String matching is fragile but necessary as PyTorch doesn't provide
+                # a specific exception type for this case
+                error_msg = str(e)
+                if "tensor subclasses" in error_msg or "Cannot access data pointer" in error_msg:
                     # Create a dummy tensor with the same shape and dtype
                     dummy_tensor = torch.zeros(tensor_value.shape, dtype=tensor_value.dtype)
                     return tvm.runtime.tensor(dummy_tensor.numpy())