[BE] [MPS] Fix `out` resize logic in `torch.where` (#121476)
By deleting `where_mps` and registering MPS dispatch for `where_kernel`.
As result of this change resizing and type-checking logic is shared between MPS, CPU and CUDA backends.
Add test_case to `TestMPS.test_where` (that should eventually be removed, when `out` OpInfo testing is enabled for MPS
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121476
Approved by: https://github.com/albanD, https://github.com/Skylion007
ghstack dependencies: #121473, #121494
