Add head_mask, decoder_head_mask, cross_head_mask to ProphetNet
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
This issue has been automatically marked as stale and been closed because it has not had recent activity. Thank you for your contributions.

If you think this still needs to be addressed please comment on this thread.<|||||>Reopened as this might still be in the works.<|||||>This PR is required for #10605.

Also, it is necessary to rebase this branch to the current `master` [As a lot of changes have been done to the repo, there are some conflicts I'm gonna handle asap.].<|||||>Hi @LysandreJik - I fixed `cross_head_mask` for this `ProphetNetModel`. At this moment, there is an error regarding the `test_forward_signature` and there is likely to be a problem with a template. These issues should be then resolved in #10605 which takes care of `cross_head_mask` for all other encoder-decoder models which have already had `head_mask` and `decoder_head_mask` merged into the master.<|||||>Update: #10605 now passes all the tests. (@LysandreJik)<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Super sorry for being incredibly slow here @stancld ! I think we can actually merge this if it passes all the tests :-)<|||||>@patrickvonplaten No worries, it's completely okay! :) I rebase this branch and now all the tests have passed.