Replace FutureMessage with ivalue::Future in RRefContext (#49960)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/49960
Test Plan: Imported from OSS
Reviewed By: lw
Differential Revision: D25730530
Pulled By: mrshenli
fbshipit-source-id: 5d54572c653592d79c40aed616266c87307a1ad8
