Add using torch::utils::Future to simplify code in RRefContext (#36811)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/36811
Test Plan: Imported from OSS
Differential Revision: D21093846
Pulled By: mrshenli
fbshipit-source-id: 61a6b1483ef1533803a18bec216ebe82aa187458
