Make `XLMRoberta` model and config independent from `Roberta`
# What does this PR do?

Removes the dependencies of `XLMRobertaConfig` and everything inside `modeling_xlm_roberta.py`. This is related to issue #19303.

I only did this for the PyTorch model as there were some models in the issue that had "PyTorch + TF" and this was not one of them. I can add for tensorflow or flax if needed!

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@sgugger 

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_The documentation is not available anymore as the PR was closed or merged._<|||||>Hi @sgugger
I ran `make style` and fixed your recommendations, but now there's an error in the `run_tests_torch`. I believe it's [this](https://app.circleci.com/pipelines/github/huggingface/transformers/48816/workflows/882fa75f-7b2f-45af-861c-1beb9881aeea/jobs/583177?invite=true#step-112-2911): 
```
OSError: Rocketknight1/esm-2-8m is not a local folder and is not a valid model identifier listed on 'https://huggingface.co/models'
If this is a private repository, make sure to pass a token having permission to this repo with `use_auth_token` or log in with `huggingface-cli login` and pass `use_auth_token=True`.
```
I don't really know how to fix that or what caused it.. any tips?