Implement sharding for MacOS jobs (#68784)
Summary:
Do not run distributed tests as part of separate shard, but keep it inside one of the two shards (to limit concurrency problems)
Fixes https://github.com/pytorch/pytorch/issues/68260
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68784
Reviewed By: seemethere, janeyx99
Differential Revision: D32653440
Pulled By: malfet
fbshipit-source-id: ebe5bbc30bdf67e930f2c766c920932700f3a4e4
