Fix RoBERTa vocab size

[amariucaitheodor]

What does this PR do?

Fixes #23388

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[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.

[amyeroberts]

Thanks for reporting and fixing with this PR! 🔥

Just two things to add before we're ready to merge:

• The style & quality checks are currently failing because the RobertaPreLayerNorm model config copies from this one. Could you run make fix-copies to propogate these changes across and push the changes?
• The docstrings of the config classes should be updated to reflect the new vocab sizes

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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