Update Jupyter Notebooks to convert MaxText checkpoint to Hugging Face

[YixuanWang-99]

Description

Add a new cell of checkpoint conversion from maxtext to huggingface in SFT and RL Jupyter Notebooks.

The main changes:

• A new cell to convert matext checkpoint to hugging face format, in both Jupyter Notebooks
• An updated config initialization in to_huggingface.py to be compatible with the new pydantic configs.

Tests

• The updated SFT and RL Jupyter Notebooks are tested on TPU v6e-8, outputs are stored in GCS

• The updated to_huggingface.py is tested using test_qwen3_to_hf.sh.

Checklist

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• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...rc/maxtext/checkpoint_conversion/to_huggingface.py	0.00%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[hengtaoguo]

LGTM!