Split RRef class into abstract RRef and RRefBase (#28942)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28942
The new abstract RRef class contains only user-facing RRef APIs.
It will be later moved to a common folder so that it can be shared
by jit and distributed packages to provide TorchScript support.
Test Plan: Imported from OSS
Differential Revision: D18240590
Pulled By: mrshenli
fbshipit-source-id: ac28cfc2c8039ab7131b537b2971ed4738710acb
