backend fallback test (#29682)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29682
This PR re-introduces backend_fallback_test.cpp, which was previously called boxed_fallback_test.cpp and showed how to use the backend fallback API.
ghstack-source-id: 94481314
Test Plan: unit tests
Differential Revision: D18462654
fbshipit-source-id: 3e9b5c8f35c05f9cd795f44a5fefd1a0aaf03509
