Making `Camembert` independent from `Roberta`, clean
This pull request is a clean version of #19312 


# What does this PR do?

related to  #19303 

Making the Camembert model (pytorch version) independent from Roberta

I have changed all the classes of camembert by copy paste from Roberta. I have done the litte necessary changes for every thing to work.

I'm still wondering how to change blocks like those, I have to  add a specific checkpoint for Camembert ? :
```python
    @add_code_sample_docstrings(
        processor_class=_TOKENIZER_FOR_DOC,
        checkpoint="deepset/roberta-base-squad2",
        output_type=QuestionAnsweringModelOutput,
        config_class=_CONFIG_FOR_DOC,
        expected_output="' puppet'",
        expected_loss=0.86,
    )
```
For testing, the following test works well :
```bash
$ RUN_SLOW=1 pytest tests/models/camembert/test_modeling_camembert.py
```

However, I have noticed the tests are only for the `CamembertModel` but not other classes like `CamembertForCausalLM` ...

## 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?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] 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?
@sgugger related to  #19303 
_The documentation is not available anymore as the PR was closed or merged._