[MPS] add support for aten::nextafter (#109685)
Fixes https://github.com/pytorch/pytorch/issues/77764#issuecomment-1722515591
Adds support for aten::nextafter to the MPS backend. Supports float and half types.
Notes:
- I've added nextafter to the output_grad_check XFAILLIST since neither this nor the cpu implementations have grad functions
- Metal Shading Language 3.1 seems to have a native nextafter() function, so once that's available, this kernel can just call that.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/109685
Approved by: https://github.com/kulinseth
