Doc improvement on DDP (#15440)
Summary:
I noticed that some users don't even know we have this support. Adding into the doc
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15440
Differential Revision: D13531045
Pulled By: teng-li
fbshipit-source-id: 9757c400c0010608758c754df04e603b36035a10
