[ProphetNet] Fix naming and wrong config
Author: patrickvonplaten
Pull request: yes

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@guillaume-be would be great if you can review here as well

This PR fixes a bad naming and wrong usage of the config parameters. Since all prophet models online have ```config.num_encoder_attention_heads==config.num_decoder_attention_heads``` this change should not lead to any problems. Luckily it was caught early on by @guillaume-be 

Fixes #9485


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Comments:
> Thank you @patrickvonplaten the changes look great! One last suggestion on my side since this PR does some renaming of the modules: I believe the naming of the `ProphetNetSelfAttention` is misleading, since it is used as a cross attention in the decoder layer:
> 
> https://github.com/huggingface/transformers/blob/02e05fb0a532e572b56ba75dad6ba3db625bbdeb/src/transformers/models/prophetnet/modeling_prophetnet.py#L1082
> 
> 
> Maybe a more appropriate name would be `ProphetNetBaseAttention` or simply `ProphetNetAttention` ?
> There is also a typo in `ProhpetNetPositionalEmbeddings` and `ProhpetNetFeedForward`

I agree with you! I should be more careful next time when naming the classes :-)