reenabling MPI test (#48725)
Summary:
fixes https://github.com/pytorch/pytorch/issues/47443.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48725
Reviewed By: mrshenli
Differential Revision: D25278758
Pulled By: walterddr
fbshipit-source-id: a02d0fef99a7941c8e98da16a45d840e12b8b0c3
