Update distributed.rst (#23289)
Summary:
Different backend is supported since https://github.com/pytorch/pytorch/pull/18595
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23289
Differential Revision: D16528229
Pulled By: soumith
fbshipit-source-id: 57753e84c015817661ba30835278ee3a899aa2d0
