[MPS] Add C++ API support for MPS backend (#96668)
- This enables the APIs `torch::mps::is_available()/synchronize()/manual_seed()` for use in PyTorch C++.
- Added test case for C++ APIs to `mps_test_allocator.cpp`
Fixes #96425
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96668
Approved by: https://github.com/kulinseth, https://github.com/albanD
