TCP init method race condition fix (#15684)

TCP init method race condition fix (#15684)

Summary:
This PR fixes a race condition for TCP init method, when master rank can exit earlier than slave ranks and thus the TCP daemon thread gets shutdown before other slaves are able to access it.

This will let every rank (process) write a special key to the store to mark that they are completed (and thus about to exit).  The master rank (who is the server) will always wait until all the ranks to complete before complete itself.

This should fix: https://github.com/pytorch/pytorch/issues/15638

Tested using the repro of https://github.com/pytorch/pytorch/issues/15638 and works fine. Also test_distributed and test_c10d should have already had this coverage.

I had to make rendezvous test in c10d the world size of 1, since it is a single process code.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15684

Differential Revision: D13570904

Pulled By: teng-li

fbshipit-source-id: 34f3bc471204bbd29320df359347ad5561c6b589