Move most custom op related tests to test_custom_ops.py (#106036)
This PR moves most custom op related tests from
test/test_python_dispatch.py to test/test_custom_ops.py. Motivation is
that I had a difficult time finding the custom op tests inside
test_python_dispatch.py.
This doesn't preserve blame, but it's OK - I'm the only person who has
really touched the moved tests so far :).
Test Plan:
- run tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106036
Approved by: https://github.com/bdhirsh, https://github.com/soulitzer
