Make RRef::LocalValue return Future (#28025)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28025
Add a PyFuture type which is wrapper of either an OwnerRRef or a
jit::Future. The difference between PyFuture and jit::Future is that
PyFuture can return an custom py::object type.
Test Plan: Imported from OSS
Differential Revision: D17936746
Pulled By: mrshenli
fbshipit-source-id: a7451af3993d98aeab462ffd5318fc6d28f915c8
