Support constant lr with cooldown
Author: LoserCheems
Pull request: yes

# What does this PR do?

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Fixes #35449
Added the 'warmup_stable_cooldown' learning rate scheduler method.
This method allows three phases, linear warmup, stable, and cooldown, where cooldown can be done using the `linear`, `cosine` and `1-sqrt` methods.



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Comments:
Thanks for the PR @LoserCheems, we already have warmup_stable_decay scheduler with `get_wsd_schedule` that does practically the same thing as the new scheduler you proposed. I think what you be good is to maybe add more options to the decay stage of the wsd_scheduler as you did in this PR (e.g linear/ 1-sqrt) WDYT ? 

> Thanks for the PR @LoserCheems, we already have warmup_stable_decay scheduler with `get_wsd_schedule` that does practically the same thing as the new scheduler you proposed. I think what you be good is to maybe add more options to the decay stage of the wsd_scheduler as you did in this PR (e.g linear/ 1-sqrt) WDYT ?

oh, thank for your suggestion, I intend to integrate different cooldown methods and minimum learning rate into `get_wsd_schedule`.
In addition, is it possible to rename `get_wsd_schedule` to `get_wsc_schedule`, which is more in line with the three phases of warmup, stable, cooldown. 

Thanks ! We can't really rename it wsc, wsd is a real term that appears in the MiniCPM paper. 

> Thanks ! We can't really rename it wsc, wsd is a real term that appears in the MiniCPM paper.

Thank you, the renaming of the function is done.

> Nice ! Just a few nits and we should be able to merge this soon ! Thanks for being reactive

Everything is ready!

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_35453). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

> LGTM ! Thanks for iterating ! Just a few nits

It's done!


@SunMarc Thank you, this PR is ready for merge.


gentle ping @ArthurZucker 

Sorry @SunMarc, is there anything wrong with this PR?