Decouples `XLMProphet` model from `Prophet` (#19406)
* decouples xlm_prophet from prophet and adds copy patterns that pass the copy check
* adds copy patterns to copied docstrings too
* restores autodoc for XLMProphetNetModel
* removes all-casing in a bunch of places to ensure that the model is compatible with all checkpoints on the hub
* adds missing model to main init
* adds autodocs to make document checker happy
* adds missing pretrained model import
* adds missing pretrained model import to main init
* adds XLMProphetNetPreTrainedModel to the dummy pt objects
* removes examples from the source-doc file since docstrings contain them already
* adds a missing new line to make check_repo happy
