[JIT] Add out-of-source-tree to_backend tests (#41145)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41145
**Summary**
This commit adds out-of-source-tree tests for `to_backend`. These tests check
that a Module can be lowered to a backend, exported, loaded (in both
Python and C++) and executed.
**Fixes**
This commit fixes #40067.
Test Plan: Imported from OSS
Reviewed By: jamesr66a
Differential Revision: D22510076
Pulled By: SplitInfinity
fbshipit-source-id: f65964ef3092a095740f06636ed5b1eb0884492d
