Fix for making student ProphetNet for Seq2Seq Distillation
Author: vishal-burman
Pull request: yes

# What does this PR do?

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Fixes # (issue)
Enables making student model of ProphetNet

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## Who can review?
@sgugger @LysandreJik 

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Models:

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Comments:
We're not actively maintaining those examples, so would need an approval from the original author (@sshleifer ) before merging

LGTM!

Thank you both!