[deepspeed docs] DeepSpeed ZeRO Inference
A good demo example has been worked out for using DeepSpeed ZeRO Inference w/o using HF Trainer [here](https://github.com/huggingface/transformers/issues/15399#issuecomment-1025240005), so let's add it to the doc as we didn't have any.

@sgugger 
_The documentation is not available anymore as the PR was closed or merged._