[Deepspeed] add support for bf16 mode
This PR:

- adds support for bfloat16 for ZeRO-1, ZeRO-2 and ZeRO-3 to HF/DS integration.
- most functional tests are now run for bf16 as well - so this PR almost doubles the number of tests. model zoo tests are left with fp16 for now, as both should work the same.
- docs are updated to document the new feature

Requirements:

1. [x] merged https://github.com/huggingface/transformers/pull/13207
2. [x] ZeRO-3 support merged https://github.com/microsoft/DeepSpeed/pull/1453
3. [x] Need new deepspeed release after the above is merged 
4. [x] update version dependency table

---------------------------

for users who want to try this early:

1. add `--bf16` to your previous fp16 or fp32 deepspeed command line 

done ;)

@sgugger 
Thanks a lot for catching those few development process leftovers, Sylvain! Much appreciating. All cleaned up now.