[deepspeed] zero inference
This PR extends HF/DS integration to support Deepspeed Zero inference. Now we don't need to waste gpu memory on allocating the optimizer/scheduler and then dropping them. And in some cases enabling what was not possible before - in case the user doesn't have the extra gpu memory and was getting OOM on inference.

Blocking events:
- [x] merged https://github.com/microsoft/DeepSpeed/pull/1514 
- [x] new deepspeed release (want v0.5.7 to be out)
- [x] update dependency table once above is done

@jeffra, @sgugger 

The CI errors seem to be irrelevant to this PR
Thanks a lot for the review and the suggestions, Sylvain. All addressed, please have another look at your convenience. Thank you.