[Deepspeed] add support for bf16 mode

[stas00]

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. merged Support for Training with BF16 #13207
2. ZeRO-3 support merged Various ZeRO Stage3 Optimizations + Improvements (including bfloat16 support) microsoft/DeepSpeed#1453
3. Need new deepspeed release after the above is merged
4. 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

[sgugger]

Thanks for working on this!

[stas00]

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

[sgugger]

Looks all good to me!