[jit] make Future type annotation available in Python (#27637)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27637
Fixes https://github.com/pytorch/pytorch/issues/26578
Test Plan: Imported from OSS
Differential Revision: D20626866
fbshipit-source-id: 20d6a3a719fddcb33e0e17a56d7123535fa20d65
