Support to_huggingface checkpoint conversion for Gemma4

[aireenmei]

Description

hf to_maxtext checkpoint conversion is already supported. Adding support for the other direction. The following changes are included to reduce frictions during the conversion:

• Previously param mapping dict won't include multimodal keys, when use_multimodal=False, this cause error if the provided maxtext ckpt contains multimodal keys, while the user only want to convert the text-related weights to hf ckpt. I improved this logic to have param_mapping always load the full dict, but keep or skip multimodal keys later depends on use_multimodal flag
• Adjust moe tile size in the scripts for 26b, so it fits the memory of v5p-8
• Add hint when scan_layers config doesn't match the ckpt

b/508269624

Tests

For gemma4-26b-a4b-it model:

• 26b/test_gemma4_to_hf.sh: the script runs to_huggingface.py for maxtext -> hf ckpt conversion, then validate the conversion by forward_pass_logit_checker.py. text-only results
• The above results are comparable to the results from convert_gemma4.sh, which runs to_maxtext.py for hf -> maxtext ckpt conversion, then validate by forward_pass_logit_checker.py text-only results

For gemma4-31b:

• 31b/test_gemma_to_hf.sh: with image results

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 133 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...rc/maxtext/checkpoint_conversion/utils/hf_shape.py	0.00%	90 Missing ⚠️
...xtext/checkpoint_conversion/utils/param_mapping.py	0.00%	23 Missing and 1 partial ⚠️
...rc/maxtext/checkpoint_conversion/to_huggingface.py	0.00%	11 Missing ⚠️
src/maxtext/checkpoint_conversion/utils/utils.py	0.00%	8 Missing ⚠️

📢 Thoughts on this report? Let us know!

[gagika]

Thanks!