[JIT] Add support for backend-lowered submodules (#41146)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41146
**Summary**
This commit adds support for using `Modules` that have been lowered as
submodules in `ScriptModules`.
**Test Plan**
This commit adds execution and save/load tests to test_backends.py for
backend-lowered submodules.
**Fixes**
This commit fixes #40069.
Test Plan: Imported from OSS
Reviewed By: ailzhang
Differential Revision: D22459543
Pulled By: SplitInfinity
fbshipit-source-id: 02e0c0ccdce26c671ade30a34aca3e99bcdc5ba7
