adding a test for ddp save()/load() (#44906)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/44906
Test Plan: Imported from OSS
Reviewed By: mrshenli
Differential Revision: D23825386
Pulled By: bdhirsh
fbshipit-source-id: 2276e6e030ef9cffd78fc78c2ffe34d60a1e160e
