[JIT] Add selective backend lowering API (#43613)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43613
**Summary**
This commit adds a helper/utility to faciliate the selective lowering of
specific submodules within a module hierarchy to a JIT backend. The reason
that this is needed is that lowering a submodule of a scripted
module to a backend after the module has been scripted requires
adjusting its JIT type.
**Test Plan**
This commit refactors `NestedModuleTest` in `jit/test_backends.py` to
use this new selective lowering API.
**Fixes**
This commit fixes ##41432.
Test Plan: Imported from OSS
Reviewed By: mortzur
Differential Revision: D23339855
Pulled By: SplitInfinity
fbshipit-source-id: d9e69aa502febbe04fd41558c70d219729252be9
