Enhance Pipe docs to explicitly mention RPC initialization. (#55187)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55187
As described in https://github.com/pytorch/pytorch/issues/54927, Pipe
docs didn't explicitly mention initializing RPC. This PR improves the docs and
also ensures Pipe throws a more useful error message when RPC is not
initialized and not an internal assertion error.
ghstack-source-id: 125563552
Test Plan:
1) unit test added.
2) waitforbuildbot
Reviewed By: rohan-varma
Differential Revision: D27521783
fbshipit-source-id: d1a5c6ca789b9a66c07a794468178c25cfd4b743
