Fix pos_mask application and update tests accordingly
Author: ferjorosa
Pull request: yes

# What does this PR do?

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Fixes #27855 (https://github.com/huggingface/transformers/issues/27855)


## Before submitting
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## Who can review?

@ArthurZucker @younesbelkada @amyeroberts

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Comments:
@ArthurZucker do you know which steps are left in the process?