⚔️ Fix bf16 fp16 config conflict issue

[yao-matrix]

w/ this PR #3515, it by default set bf16 set to True regardless what fp16 setting is. In case where user set fp16 explicitly to True(as in ut case here https://github.com/huggingface/trl/blob/main/tests/slow/test_dpo_slow.py#L132), there will be error message "At most one of fp16 and bf16 can be True, but not both".

Fixing this issue by setting bf16 to True only when fp16 is False and bf16 is not set.

@qgallouedec , pls help review, thx.

[qgallouedec]

Yes thank you, this PR makes sense. By the way, that's why slow tests fail on main.

The only thing that bothers me is that None by default isn't very informative, maybe even misleading. For the moment, I don't really see an alternative.

[yao-matrix]

Yes thank you, this PR makes sense. By the way, that's why slow tests fail on main.

The only thing that bothers me is that None by default isn't very informative, maybe even misleading. For the moment, I don't really see an alternative.

@qgallouedec Yes, since it's a post_init_, and we need to know whether the bf16 is set explicitly by users or just a automatic default, so the best way i can figure to set it as optional w/ default None. Yes, it's not informative, maybe we can enhance docstring to improve that?

Like "Whether to use bf16 (mixed) precision instead of 32-bit. Requires Ampere or higher NVIDIA "
"architecture or Intel XPU or using CPU (use_cpu) or Ascend NPU. If user doesn't set it explicitly and fp16 not set to True, we default set bf16 to True. This is an experimental API and it may change."

WDYT?

[qgallouedec]

Yes, it's not informative, maybe we can enhance docstring to improve that?

Yes, I think that's the best way to go at this stage. I'm fine with what you suggested, just delete this

This is an experimental API and it may change.

[qgallouedec]

I just realized that I asked you to remove something that was already in the doc. But still, can you remove it? I think bf16 is stable enough now

[HuggingFaceDocBuilderDev]

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