Cache intermediate results in SquadProcessor
Author: mzilinec
Pull request: yes

# What does this PR do?

SQuAD preprocessing takes a very long time and may result in OOM (RAM) for large Squad-format datasets, such as the ones in LegalBench or [CUAD](https://github.com/TheAtticusProject/cuad).

There are more things that can be improved, but here I only fixed some repetitive work in `transformers.data.processors.squad.SquadProcessor`, which runs a for loop on each character in a string of length ~10,000+. Caching this computation brings my wall time down from ~5 minutes to ~5 seconds on the CUAD dataset. As far as I know this doesn’t change any logic.

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## Who can review?

@LysandreJik

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Comments:
This script is mostly here as an example and we've seen in the past it's quite easy to get it wrong and have a model that does not end up training. I think the PR is great to keep for reference, but I don't think we'll want to merge it into main for now.

Maybe cc @SunMarc or @muellerzr if you have the bandwidth to look into this

Thanks!

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