Added implementation and tests for MPS Hardswish (#87952)
## What?
Fixes issue #86807 by adding MPS backend support for aten::hardswish.
## How?
Registered mps hardswish functions in native_functions.yaml, and added the code implementation to Activations.mm.
Added functions:
- hardswish_mps
- hardswish_mps_
- hardswish_backward_mps
- hardswish_out_mps
## Testing
Added test in test/test_mps.py and tested code using the command `python3 test/test_mps.py -k test_hardswish`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87952
Approved by: https://github.com/kulinseth, https://github.com/kit1980
