Enable mask for AdamW optimizer

[RissyRan]

Description

Enable mask for AdamW optimizer

• Add mask option for AdamW optimizer
• Rename muon_test as optimizers_test, and add unit tests for mask generation

Tests

• Add unit tests accordingly
• Expect all github runners are green

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.

[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 introduces a mechanism to exclude specific parameters from AdamW weight decay based on regular expression patterns defined in the configuration. The implementation is well-tested and neatly integrated into get_optimizer and the custom adam_pax optimizer.

🔍 General Feedback

• The logic accurately matches the optax API expectations for optimizer masks.
• I've left a small optimization suggestion to pre-compile the regular expressions during mask initialization, which will improve the tracing time and fail fast on invalid patterns.

[codecov]

Codecov Report

❌ Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/optimizers/optimizers.py	73.33%	4 Missing ⚠️

📢 Thoughts on this report? Let us know!

[gagika]

looks good 2 minor comments:

[gagika]

thanks