[doc] bf16/tf32 guide
This PR adds performance-specific docs as a follow up to https://github.com/huggingface/transformers/pull/13207

- adds the missing `--fp16_full_eval`
- adds `--bf16` and `--bf16_full_eval`
- adds discussion of `tf32`

This is the first pass, surely will add more down the road.

@sgugger 
Thanks a lot for the proofreading and the corrections, @manuelciosici 

You couldn't make suggestions since the PR was merged already.

I applied your corrections to this ongoing PR https://github.com/huggingface/transformers/pull/14580 that collects small fixes.

I even figured out how to make the right attribution to your suggestions ;)

```
git commit --author "Manuel R. Ciosici <manuelrciosici@gmail.com>" -ma "corrections"
```