[Docs][BE] DDP doc fix (#71363)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71363
Looks like DDP example is currently broken as per
https://discuss.pytorch.org/t/official-ddp-example-is-broken/141493. Fix the
issue by setting the correct env variable.
ghstack-source-id: 147080377
Test Plan: CI
Reviewed By: mrshenli
Differential Revision: D33607250
fbshipit-source-id: e0e7d03cc365c186253b959c4c5405a5e3609218
(cherry picked from commit 32472884ec04d0e9b348b07d645dd1027389f8e8)
