Add the option to avoid loading the checkpoint twice

[xuefgu]

Description

Add the option to avoid loading the checkpoint twice.

It's protected with the load_checkpoint_only_once flag.

Tests

Manually tested.

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 20 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/MaxText/rl/train_rl.py	0.00%	20 Missing ⚠️

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