RL: Add chips_per_vm hyperparameter and fix an issue of missing config

[charlesli640]

Description

Fix b/471046638
Fix an issue of missing config in types.py

Tests

Follow the tutorial doc to run Reinforcement Learning on single-host TPUs. Need this change to make it working.

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 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/MaxText/rl/train_rl.py	0.00%	1 Missing ⚠️

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[A9isha]

Thanks Charles!