Support Tensor.is_alias_of (#3966)
Summary:
Tensor.is_alias_of relies on Storage to perform. However, XLATensorImpl was
not implemented with that in mind. This commit adds a fake storage to XLATensor
as a marker to mark XLATensor that point to the same storage. The reason
why it's not done at XLATensorImpl is that XLATensor maintains the view ops/alias
logic in XLATensor class instead of relying on XLATensorImpl to do the check.
Test Plan:
./test/cpp/build/test_ptxla --gtest_filter=AtenXlaTensorTest.TestViewIsAliasOf
