Allow text generation for ProphetNetForCausalLM

[guillaume-be]

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 #9702

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Who can review?

@patrickvonplaten

[patrickvonplaten]

Thanks a lot for fixing it @guillaume-be