basic SymInt test for functionalization (#80418)
`expand` is one of a handful of ops with SymInt support today, so this PR gives a basic test that shows functionalization properly mapping `expand.SymInt` -> `expand_copy.SymInt`. I added the logic to handle this properly in https://github.com/pytorch/pytorch/pull/80251, but didn't add a test for it. (see the [code](https://github.com/pytorch/pytorch/pull/80251/files#diff-da7d91d9e59774e3ee8d120a0f97e52058b73125fd7edd55b5c2e71d4ce5629dR330))
I want to add a more comprehensive test that also shows something more E2E (using `PySymInt`'s to avoid baking in shapes, running functionalization, and fx-tracing the output to show that functionalization ran properly), but I think it's currently blocked on some other work.
At least today, `FakeSymbolicTensor` doesn't play well with `make_fx` (but @Chillee mentioned - should it?)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80418
Approved by: https://github.com/ezyang, https://github.com/albanD
