text: 
  
  TITLE
  Add head_mask, decoder_head_mask, cross_head_mask to ProphetNet
  
  COMMENTS
  8
  
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  BODY
  This PR implements `head_mask`, `decoder_head_mask` and `cross_head_mask` for ProphetNet (and Longformer as there's a copy dependency) and it is the follow-up to the open issue #9814. 
  
  **Motivation:** This PR is a part of an endeavour to enable the usage of `head_mask` and `decoder_head_mask` for all encoder-decoder transformers following the recent work on BART-like models (#9569).
  
  <hr>
  
  Fixes: https://github.com/huggingface/transformers/issues/9814
  
  Reviewers: @patrickvonplaten