Add the Runtime components for MPS backend. (#76725)
The PR adds the runtime components and few basic operations like copy, as_strided for MPS backend.
Current list of identified TODOs are:
- https://github.com/pytorch/pytorch/issues/77176
- Unify the logic with CUDACachingAllocator and remove redundant code.
- https://github.com/pytorch/pytorch/issues/77170
- Look into using C++ smart pointers where possible with ObjC code
- Use empty_strided_generic() to implement the `empty_strided_mps` code
- https://github.com/pytorch/pytorch/issues/77144
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76725
Approved by: https://github.com/albanD
