Tutorial combining DDP with Pipeline Parallelism to Train Transformer models (#1347)

* Tutorial combining DDP with Pipeline Parallelism to Train Transformer models.

Summary: Tutorial which places a pipe on GPUs 0 and 1 and another Pipe
on GPUs 2 and 3. Both pipe replicas are replicated via DDP. One process
drives GPUs 0 and 1 and another drives GPUs 2 and 3.

* Polish out some of the docs.

* Add thumbnail and address some comments.

Co-authored-by: pritam <pritam.damania@fb.com>

Author: pritamdamania87