Do distributed init after data loading

Summary:
FACEBOOK

This switches back to torch.multiprocessing.spawn, instead of directly calling fb_train.par using a subprocess.Process. This has the advantage that exceptions are propagated properly. It also moves the distributed_init part to happen after data loading, which gets around the timeout issue.

The downside of this approach is that it's not so easy to pipe stdout to multiple places, which was nice when using the sweep.py scripts. I'm still working on a fix for that.

Reviewed By: rutyrinott, ngoyal2707

Differential Revision: D13873224

fbshipit-source-id: 08d593233b8d23590c01c723363630a79804a8b0