Decouples `XLMProphet` model from `Prophet` 
@sgugger , 

Per the issue #19303, the `Prophet` model dependency is removed from `XLMProphet` and it now directly inherits from `PretrainedModel`.  

- [As discussed in a different PR review](https://github.com/huggingface/transformers/pull/19346#discussion_r988069210) , I've moved some of the docstring examples to inside the corresponding docs/source location.  Let me know if there are some tests you want me to run. 

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      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.
- [x] 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?

Thanks for reviewing the PR!
_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger  - the requested changes have all been done. Thanks for your review! 