[MPS] Add support for complex scalars (#119318)
- Switch to native complex support if running on MacOS Monterey or newer for binary ops.
- Python complex scalars are always represented in PyTorch as ComplexDouble, but MPS yet to support double precision types, so downcast them to floats
- Also add `cf`(for complex float) and `ch`(for complex half) to MPSScalar value union
- Fix complex scalars to view promotion, by introducing `legacy_complex_as_view` helper function, that non-float types to complex and promotes CPU complex scalars to MPS before turning them into a view.
- Add `test_tensor_scalar_binops`
Fixes https://github.com/pytorch/pytorch/issues/119088
Test plan: CI (have quite a lot of tests, see new unexpected successes) + `python -c "import torch;x,y=torch.rand(2, 2, dtype=torch.cfloat, device='mps'),torch.tensor(2+3j,dtype=torch.chalf);print(y+x)"`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119318
Approved by: https://github.com/albanD
