Adding support for tokens being suffixes or part of each other.
Author: Narsil
Pull request: yes

# What does this PR do?

Fixes issues with reported utilization of protein tokens, where some special tokens
might be part of other tokens, of suffixes of each other.

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Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
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## Who can review?

@LysandreJik
@sgugger

Ran slow tests on same box, and got the same results as last time.

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

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