Fix link in distributed contributing doc and add link (#49141)
Summary:
One of the links for ramp up tasks wasn't showing any results and the other was only RPC results. Instead of this, I just changed it to one link that has `pt_distributed_rampup` which seems reasonable as the developer will be able to see both RPC and distributed tasks.
Also added test command for DDP tests.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49141
Reviewed By: ezyang
Differential Revision: D25597560
Pulled By: rohan-varma
fbshipit-source-id: 85d7d2964a19ea69fe149c017cf88dff835b164a
