add functions to inspect model and optimizer status to trainer.py
Author: CKeibel
Pull request: yes

# What does this PR do?

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Add functions `get_num_trainable_parameters` to return the number of parameters which require grad, `get_learning_rates` to return the learning rates of parameter groups and `get_optimizer_group` to return optimizer groups for parameters.

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Fixes #29016 


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Models:

- text models: @ArthurZucker and @younesbelkada
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@amyeroberts @muellerzr 

Comments:
> Very nice PR! Any chance you could add some basic tests in `tests/trainer/test_trainer.py` for these?

Of course, I wasn't sure where to add them, but now I see that I missed that `test_trainer.py` exists. I will add the tests later. :)

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_29838). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Great, thanks! I'll give a ✅ after, great job and very clean implementation :) 

Is there anything else to do? :)  @amyeroberts @muellerzr 

@CKeibel Nope - merging now! 

Excellent thanks, then I'll start looking for a new issue to work on. :)

Looking forward to the future PRs :) Thanks again for contributing to improving the library!