Allow specifying trainer tensor parallelism in multislice RL

[xuefgu]

Description

Allow specifying trainer tensor parallelism in multislice RL

FIXES: b/480979614

Tests

Manually tested.

Checklist

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• 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 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/MaxText/rl/train_rl.py	0.00%	9 Missing ⚠️

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