Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30066
This commit adds design reasoning and walks through four scenarios
for RRef.
Test Plan: Imported from OSS
Reviewed By: rohan-varma
Differential Revision: D18595094
Pulled By: mrshenli
fbshipit-source-id: 134102901ce515a44a2e7cd013b62143a6158120
