[DeepSpeed] fp32 support
Things we need to sync with the upcoming `deepspeed==0.3.16` release:

- `zero.Init` now takes a config as an argument
- fp32-support integration, plus doc and tests
- start troubleshooting section

### Future TODO

will probably do in the next PR:
- switch `from_config()` to perform the same `zero.Init` as `from_pretrained` + add test.

###  Blocking events

PRs waiting to be merged before this PR can be merged:

- [x] https://github.com/microsoft/DeepSpeed/pull/1008 `zero.Init(config=ds_config)` new arg 
- [x] https://github.com/microsoft/DeepSpeed/pull/1004 fp32 support
- [x] new release is needed 0.3.16

@sgugger 
That's correct. 

Earlier I tried to use `cur_version>pre_version` so it'd already work with master version, but then people were reporting bugs because they were on some older master version, so while this is less convenient, it avoids invalid bug reports and saves time to all ;)

 