Add MPS merge rules (#78259)
This enables MPS PR reviewed by @kulinseth and @razarmehr to be merged into default branch using mergebot.
"Lint" is the only mandatory check for this rule, as MacOS and iOS builds are on trunk-only for now
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78259
Approved by: https://github.com/albanD, https://github.com/atalman, https://github.com/janeyx99, https://github.com/seemethere
