Restore CPU distributed tests (#97424)
This looks pretty stable now on [HUD](https://hud.pytorch.org/reliability/pytorch/pytorch?jobName=unstable%20%2F%20linux-focal-py3.8-gcc7%20%2F%20test%20(distributed)), so moving it back from unstable. Pending some discussion on https://github.com/pytorch/pytorch/issues/97178
I'll monitor this a bit longer before merging.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97424
Approved by: https://github.com/clee2000
