rename join_rpc to wait_all_workers in public api (#30050)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30050
Renames this API to wait_all_workers as discussed.
ghstack-source-id: 94273005
Test Plan: Unit tests pass
Differential Revision: D18581466
fbshipit-source-id: 4ff5d5fb2d528f17252d5b5f30c3047d2efb92bf
