Pull in fairscale.nn.Pipe into PyTorch. (#44090)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44090

This is an initial commit pulling in the torchgpipe fork at
https://github.com/facebookresearch/fairscale.

The purpose of this commit is to just pull in the code and ensure all tests and
builds work fine. We will slowly modify this to match our intended API
mentioned in https://fb.quip.com/txurAV3zIFox#RPZACAfAKMq. Follow up PRs would
address further changes needed on top of the initial commit..

We're pulling the code into the `torch.distributed._pipeline.sync` package. The
package is private on purpose since there is a lot of work (ex: docs, API
changes etc.) that needs to go in before we can actually officially support
this.
ghstack-source-id: 114864254

Test Plan:
1) waitforbuildbot
2) Ran all tests on my devgpu

Reviewed By: mrshenli

Differential Revision: D23493316

fbshipit-source-id: fe3c8b7dadeeb86abdc00e8a8652491b0b16743a

LICENSE

@@ -16,23 +16,26 @@ Copyright (c) 2016-present, Facebook Inc. All rights reserved.
 
 All contributions by Facebook:
 Copyright (c) 2016 Facebook Inc.
- 
+
 All contributions by Google:
 Copyright (c) 2015 Google Inc.
 All rights reserved.
- 
+
 All contributions by Yangqing Jia:
 Copyright (c) 2015 Yangqing Jia
 All rights reserved.
- 
+
+All contributions by Kakao Brain:
+Copyright 2019-2020 Kakao Brain
+
 All contributions from Caffe:
 Copyright(c) 2013, 2014, 2015, the respective contributors
 All rights reserved.
- 
+
 All other contributions:
 Copyright(c) 2015, 2016 the respective contributors
 All rights reserved.
- 
+
 Caffe2 uses a copyright model similar to Caffe: each contributor holds
 copyright over their contributions to Caffe2. The project versioning records
 all such contribution and copyright details. If a contributor wants to further