Consolidate legacy checkpoint scripts into folder

[RissyRan]

Description

Move all legacy checkpoint scripts from folder src/MaxText into folder src/MaxText/utils/ckpt_scripts to align with new checkpoint utility src/MaxText/utils/ckpt_conversion. We have plan to migrate all legacy scripts into conversion utility in Q1 2026 timeline.

This PR helps us to consolidate all scripts, and resolves b/452391826.

Tests

• Expect all tests passing
• Successfully verified a small model with new path gemma3-4b - link

export CHKPT_BUCKET=gs://maxtext-gemma/gemma3/flax
python3 -m MaxText.utils.ckpt_scripts.convert_gemma3_chkpt --base_model_path ${CHKPT_BUCKET}/4b --maxtext_model_path gs://runner-maxtext-logs/ranran/gemma3 --model_size 4b

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• 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.

[github-actions]

🤖 Hi @RissyRan, I've received your request, and I'm working on it now! You can track my progress in the logs for more details.

[github-actions]

📋 Review Summary

This Pull Request successfully consolidates legacy checkpoint scripts into a new src/MaxText/utils/ckpt_scripts folder. The changes are well-executed, primarily involving path updates across documentation, shell scripts, and Python import statements.

🔍 General Feedback

• All file renames and path updates are consistent and correctly reflect the new directory structure.
• The changes align with the stated goal of organizing checkpoint utilities.

[shuningjin]

Do we also move dequantize_mxfp4.py here, which we use for gpt-oss dequantization?

• It won't be a legacy script though. I see you moved deepseek_fp8_to_bf16.py.

maxtext/src/MaxText/scratch_code/dequantize_mxfp4.py

Line 19 in 341061f

python3 dequantize_mxfp4.py --input-path=<input_path> --output-path=<output_path>

[RissyRan]

dequantize_mxfp4

Good call! I will move that together.

[hengtaoguo]

LGTM!

[parambole]

LGTM!