non-identical position_ids in an input batch
Author: hmohebbi
Pull request: yes

# What does this PR do?

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The possibility of defining different position_ids for each input example through a batch.
This is useful when we wish to pad some tokens in the input while simultaneously removing them to improve efficiency.


## 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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      Pull Request section?
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- [ ] Did you write any new necessary tests?


## Who can review?

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@LysandreJik 
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