[JIT] Add out-of-source-tree to_backend tests (#40842)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40842
**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
Differential Revision: D22418731
Pulled By: SplitInfinity
fbshipit-source-id: 621ba4efc1b121fa76c9c7ca377792ac7440d250
