[Typing] Export `torch.backends` as subpackage (#102099)
So that `pyright` is happy.
Do a little refactor in `mps/__init__.py` to avoid cyclical dependency on `torch.fx` by calling `mps._init()` implicitly.
Fixes https://github.com/pytorch/pytorch/issues/101686
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102099
Approved by: https://github.com/Skylion007
