[deepspeed] Move code and doc into standalone files
Deepspeed integration is no longer bound to HF Trainer and its docs have grown too big to be a subsection of the Trainer docs. This PR:
- creates `transformers.deepspeed` and migrates all the code and references to it
- moves docs to `deepspeed.rst`

Note to @sgugger - I tried to make this easy to review by not making any changes to any content of code or text. Other then the updated imports in the code, the only change is the preamble section of `deepspeed.rst` - there is no need to re-review the rest unless you'd like to. I flagged that new text below.

And I also added a poor-man style redirect links from most of the previous sections in `trainer.rst` so that the old links still work. Well, had to add anchors to sections in the new doc for this to work.

@sgugger 