Tentative improvement on sequence_length error for position_ids
Author: Narsil
Pull request: yes

# What does this PR do?

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Currently the failure is `index out of range in self`.  (it fails in pytorch position_embeddings call).
The current PR aims to simply improve the error message by saying that the position_ids are simply too large.

It does add an `if` statement and a `max` call in a forward pass which is not so great 
but it might help other users understand better the failure mode.

If this PR is desirable (which I am not sure it is because it has a non zero overhead), we should probably look at all
position_ids and copy the behavior at some point.



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Comments:
I'm going to close this PR. I started it but ultimately feel it's not worth it.
The overhead is non zero for a simple error message, that won't bring that much value (most users can debug anyway, users that can't won't necessarily do a better job with the improved error message)