[MPS] add support for sgn to MPS backend (#110829)
Fixes #86805
Adds support for sgn to MPS backend.
Notes:
1. @malfet self-assigned this when he was working on implementing polar, but from what I can tell, he didn't end up needing to implement it.
2. @Berzeg implemented this last year, before view_as_complex was supported. Because of @malfet recent contributions, however, @Berzeg 's implementation works. I've removed the part of his implementation that dealt with non-complex dtypes (since these can just be passed to at::sign), matched the more recent pattern we've been using in UnaryOps.mm, and thrown in a simple implementation of _efficientzerotensor for mps, so that the backward function works.
3. @Berzeg deserves a good bit of credit for this, so let me know if there's a way to assign him some without jamming up the pr (he seems to be AWOL since last working on this)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110829
Approved by: https://github.com/malfet
