Document BLOOM lm_logits original training behavior
# What does this PR do?

~Compute softmax of Bloom in fp32 during half percision~

Document BLOOM lm_logits original training behavior

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
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      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@thomasw21 @stas00 @TevenLeScao
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20027). All of your documentation changes will be reflected on that endpoint.<|||||>cc @thomasw21 and @younesbelkada but I don't think this is necessary as bfloat16 is more numerically stable than float16?<|||||>we noticed considerable performance difference between softmax in bf16 and softmax in fp32 in an internal bloom-based model. since during training the softmax is conducted in fp32, this change better reflects the model behavior in megatron-deepspeed.<|||||>@shijie-wu can you please share a bit more about how you run the model? <|||||>We redid the experiment regarding fp32 in SA. It seems like the gap we observed earlier is caused by some other issues. I have removed that part of the PR. After discussion with @thomasw21, instead of enforcing a type conversion, this PR will instead document the original behavior so that advanced users could recover it.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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