[TorchScript] C++ interface of to_<backend> (#51797)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51797
The C++ API, ```codegen_backend_module``` is added to ```to_<backend>```. Python related stuffs are decoupled in this function. It can be used from both C++ and python.
* Tests
Python: The existing ```test_backends.py```, which calls the C++ API under the hood.
C++: The end-to-end test of ```jit.BackendTest.ToBackend``` is added in ```test_backend.cpp```. The original class definitions in this file is moved to ```test_backend_lib.cpp```
ghstack-source-id: 121687464
(Note: this ignores all push blocking failures!)
Test Plan: CI
Reviewed By: raziel
Differential Revision: D26280518
fbshipit-source-id: fd466e4b448847ce64010a3297fff0b5760c5280
