Allow text generation for ProphetNetForCausalLM 
# What does this PR do?

The configuration for ProphetNetForCausalLM is overwritten at initialization to ensure that it is used as a decoder (and not as an encoder_decoder) for text generation.

The initialization of the parent class for ProphetNetForCausalLM is done before this overwrite, causing the `model.config.is_encoder_decoder` to remain possibly True. This leads to an error if the generate method of the model is later called as the non-existing method `get_encoder` is called.

Fixes https://github.com/huggingface/transformers/issues/9702


## 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/master/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.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?
@patrickvonplaten

Thanks a lot for fixing it @guillaume-be 