[FSDP] customizable backend in test (#67135)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67135
Add ability to use env var backend for quicker testing (and gloo2 in
the future)
ghstack-source-id: 142274304
Test Plan: CI
Reviewed By: mrshenli
Differential Revision: D31878285
fbshipit-source-id: 80ae7107cd631a1a15ebc23262b27d8192cfe4b6
