Add xibinliu to code owner list

[xibinliu]

Description

For:

• .github/workflows/:
  • CI changes (CI speed-up)
• *:
  • general pre-train / post-train pass changes (NNX migration)
• src/maxtext/layers, src/maxtext/models:
  • general decoder, model, and layer changes (NNX migration)

Tests

NA

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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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