[fix] cannot import name 'Pop2PianoFeatureExtractor' from 'transformers'
Author: faaany
Pull request: yes

## What does this PR do?
I have all the necessary third-party libraries installed in my environment. But with the following command:
```bash
python -m pytest -rA tests/models/pop2piano/test_feature_extraction_pop2piano.py::Pop2PianoFeatureExtractionTest::test_attention_mask
```
I got the following import error: 
```bash
================================================================= ERRORS =================================================================
______________________________ ERROR collecting tests/models/pop2piano/test_feature_extraction_pop2piano.py ______________________________
ImportError while importing test module '/.tests/transformers/tests/models/pop2piano/test_feature_extraction_pop2piano.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib/python3.10/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/models/pop2piano/test_feature_extraction_pop2piano.py:48: in <module>
    from transformers import Pop2PianoFeatureExtractor
E   ImportError: cannot import name 'Pop2PianoFeatureExtractor' from 'transformers' (/.tests/transformers/src/transformers/__init__.py)
```
This PR fixes the above error by following the `__init__.py` file of the musicgen_melody model and below is the test result after fix:
```bash
======================================================== short test summary info =========================================================
PASSED tests/models/pop2piano/test_feature_extraction_pop2piano.py::Pop2PianoFeatureExtractionTest::test_attention_mask
=========================================================== 1 passed in 3.53s ============================================================
```

## Who can review?
@ydshieh 

Comments:
Hi @faaany Thanks a lot for catching this and the PR.

The init stuff has been changed and now it takes a different form. Could you first revert the current changes of this PR, then add lines like

> __all__ = ["Pop2PianoFeatureExtractor"]

and

> __all__ = ["Pop2PianoProcessor"]

and

> __all__ = ["Pop2PianoTokenizer"] 

at the end of the corresponding files?

See the ending line o
> src/transformers/models/pop2piano/configuration_pop2piano.py

for example.

And let me know if it resolves the issue 🙏 


> Hi @faaany Thanks a lot for catching this and the PR.
> 
> The init stuff has been changed and now it takes a different form. Could you first revert the current changes of this PR, then add lines like
> 
> > **all** = ["Pop2PianoFeatureExtractor"]
> 
> and
> 
> > **all** = ["Pop2PianoProcessor"]
> 
> and
> 
> > **all** = ["Pop2PianoTokenizer"]
> 
> at the end of the corresponding files?
> 
> See the ending line o
> 
> > src/transformers/models/pop2piano/configuration_pop2piano.py
> 
> for example.
> 
> And let me know if it resolves the issue 🙏

Hi @ydshieh , thanks for the quick response. No, the error persists.  

Thanks for checking. I will have to debug why it is not working with the new way of `__init__` (unless you want to dive into this topic!)

> Thanks for checking. I will have to debug why it is not working with the new way of `__init__` (unless you want to dive into this topic!)

ok, let me look into it and update my PR.  

> Thanks for checking. I will have to debug why it is not working with the new way of `__init__` (unless you want to dive into this topic!)

I misunderstood your suggestion. you are right. It works now! Code upaded.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_35604). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Hi folks, any feedback? @Rocketknight1 @ArthurZucker @yonigozlan 

@faaany You don't need to update with `main` while waiting for a final review from the core maintainers :-). We can do it at the end just before merge

(they are busy at this moment)

The fix is trivial. Do you think we can proceed? Thx a lot! 

@zucchini-nlp 

Maybe you can help to unblock here?