[Flax] Correct shift labels for seq2seq models in Flax
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Fixes #12719 

This PR makes sure that the `shift_tokens_right` is always written in numpy as it will always be called in the data-collator


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Models:

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