Remove rpc.sync_rpc from the public API. (#30033)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30033
Removing this API for now since we don't have a concrete use-case for
this yet and as a result exposing this as a public API might result in users
depending on this API.
We can always add some variant of this API back if needed later.
ghstack-source-id: 94138302
Test Plan: waitforbuildbot
Differential Revision: D18578056
fbshipit-source-id: 078c62331725e03bd5702624afc16b1cdcdf26a4
