Fix: Add a INFO-level log when fallback to gpt2tokenizer

[halogen22]

Summary

To improve user noticefication when fallback to gpt2tokenizer, I added INFO logs to the following script. dify/api/core/model_runtime/model_providers/__base/tokenizers/gpt2_tokenzier.py Fixes #12486

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Checklist

[!IMPORTANT]
Please review the checklist below before submitting your pull request.

• [ ] This change requires a documentation update, included: Dify Document
• [x] I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• [x] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [x] I've updated the documentation accordingly.
• [x] I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods