[JIT] Add support for backend-lowered submodules (#40841)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40841
**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
Differential Revision: D22418716
Pulled By: SplitInfinity
fbshipit-source-id: d2b2c6d5d2cf3042a620b3bde7d494f1abe28dc1
