rename init_model_parallel to init_rpc (#29762)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29762
Rename this API as discussed, since it's use cases extend beyond only
model parallelism.
ghstack-source-id: 94020627
Test Plan: Unit tests pass
Differential Revision: D18491743
fbshipit-source-id: d07676bb14f072c64da0ce99ee818bcc582efc57
