Fix slow GemmaTokenizer and improve SPM slow -> fast conversion process
Author: xenova
Pull request: yes

# What does this PR do?

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Fixes # (issue)


## Before submitting
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Comments:
cc @ArthurZucker 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_32191). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

cc @ArthurZucker 

~Failing tests seem unrelated? 🤔~ [Bug](https://discuss.circleci.com/t/retry-from-failed-incompatible-with-workflow-continuation/43167) in circle CI. Pushing an empty commit to trigger everything again fixed it.