[CMake] Add functorch target (#83464)

Move functorch/functorch into `functorch` folder
- Add functorch/CMakeLists.txt that adds `functorch` native python exension
- Modify `setup.py` to package pytorch and functorch together into a single wheel
- Modify `functorch.__version__` is not equal to that of `torch.__version__`
- Add dummy `functorch/setup.py` file for the projects that still want to build it

Differential Revision: [D39058811](https://our.internmc.facebook.com/intern/diff/D39058811)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83464
Approved by: https://github.com/zou3519

MANIFEST.in

@@ -26,5 +26,6 @@ recursive-include benchmarks *.*
 recursive-include scripts *.*
 recursive-include mypy_plugins *.*
 recursive-include modules *.*
+recursive-include functorch *.*
 prune */__pycache__
 global-exclude *.o *.so *.dylib *.a .git *.pyc *.swp