[Bugfix] Allow pinning of empty tensor (#6044)

python/dgl/backend/pytorch/tensor.py

@@ -451,6 +451,8 @@ else:
 
 
 def zerocopy_to_dgl_ndarray_for_write(input):
+    if input.numel() > 0:
+        # only check non-empty tensors
         assert input.is_contiguous(), (
             "Cannot convert non-contiguous tensors "
             "to dgl ndarray for write. Call .to_contiguous() first."

tests/python/pytorch/utils/test_pin_memory.py

@@ -31,6 +31,10 @@ def test_pin_view():
     with pytest.raises(dgl.DGLError):
         dgl.utils.pin_memory_inplace(v)
 
+    # make sure an empty view does not generate an error
+    u = t[10:10]
+    u = dgl.utils.pin_memory_inplace(u)
+
 
 @pytest.mark.skipif(
     F._default_context_str == "cpu", reason="Need gpu for this test."