[GraphBolt][CUDA] Hot fix after inplace pinning PR (#6974)

python/dgl/graphbolt/impl/fused_csc_sampling_graph.py

@@ -34,16 +34,17 @@ class FusedCSCSamplingGraph(SamplingGraph):
     ):
         super().__init__()
         self._c_csc_graph = c_csc_graph
-        self._is_inplace_pinned = set()
 
     def __del__(self):
         # torch.Tensor.pin_memory() is not an inplace operation. To make it
         # truly in-place, we need to use cudaHostRegister. Then, we need to use
         # cudaHostUnregister to unpin the tensor in the destructor.
         # https://github.com/pytorch/pytorch/issues/32167#issuecomment-753551842
+        if hasattr(self, "_is_inplace_pinned"):
             for tensor in self._is_inplace_pinned:
                 assert (
-                torch.cuda.cudart().cudaHostUnregister(tensor.data_ptr()) == 0
+                    torch.cuda.cudart().cudaHostUnregister(tensor.data_ptr())
+                    == 0
                 )
 
     @property
@@ -990,6 +991,8 @@ class FusedCSCSamplingGraph(SamplingGraph):
         # cudaHostUnregister to unpin the tensor in the destructor.
         # https://github.com/pytorch/pytorch/issues/32167#issuecomment-753551842
         cudart = torch.cuda.cudart()
+        if not hasattr(self, "_is_inplace_pinned"):
+            self._is_inplace_pinned = set()
 
         def _pin(x):
             if hasattr(x, "pin_memory_"):