Improve `distributed.get_rank()` API docstring (#63296)
Summary:
See discussion in https://pytorch.slack.com/archives/CBHSWPNM7/p1628792389008600
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63296
Reviewed By: cbalioglu
Differential Revision: D30332042
Pulled By: mrshenli
fbshipit-source-id: 3a642fda2e106fd35b67709ed2adb60e408854c2
