polish examples in docstrings and update docs to reflect correct use of (#30052)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30052
Some of the examples provided in `rpc/api.py` were not updated along
with the code changes, this PR updates them. Also removes the
`dist.ProcessGroup` information since `init_rpc` now initializes a default
process group.
ghstack-source-id: 94273004
Test Plan: Unit tests pass
Differential Revision: D18582596
fbshipit-source-id: a637683f0221f9600f7e50b74e9f7e5a1d331d8f
