Add failing test for ProphetBet batching
Following suggestion by @patrickvonplaten I created a failing test for this issue https://github.com/huggingface/transformers/issues/13612. It's a little over my head but I think this is what he meant.

I dunno if putting it as a `@slow` test under integration tests was the right choice. But, it seems to work. I sanity checked that this same test passes with T5 and BART (but didn't add the test for them).
Hey @deklanw - thanks for the test! It currently fails no? <|||||>@patrickvonplaten yep, it fails

<|||||>Ok, I think we should try to fix it in this PR then as well :-) I'm a bit under-water at the moment, but I can try to fix it in like 2 weeks<|||||>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.

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