Fix ProphetNet Flaky Test
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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This PR aims at solving the flaky prohpetnet test: https://app.circleci.com/pipelines/github/huggingface/transformers/21170/workflows/749ec532-0847-4d1b-8078-ca27bfdbe318/jobs/182387 . I double-checked the code and everything looks correct. Also, I've ran the test 100 times locally with the increased tolerance to somewhat make sure that it fixes the flaky CI

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Fixes # (issue)


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Models:

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Comments:
I may be mistaken, but this test has appeared since the ProphetNet refactor, right? Is it due to that refactor, or is it a newly added test?