Add docs for RPC, dist autograd, and RRef modules (#29276)
Summary:
Closes https://github.com/pytorch/pytorch/issues/28983. Documentation for `torch.distributed.rpc` and `torch.distributed.autograd` modules. Also fixes/tidies up some of the docstrings in rpc/autograd, and moves some functions to be private so they don't show up in the documentation.
Note: Much of the text to describe/explain the RPC/RRef layers are taken from the following RFCs: https://github.com/pytorch/pytorch/issues/23110, https://github.com/pytorch/pytorch/issues/26759
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29276
Differential Revision: D18478754
Pulled By: rohan-varma
fbshipit-source-id: e9a7089baf5275304e5408d319eb9bf98e53fff8
