Add option to start test_batch in train_rl from a specific index, also add default tokenizer_path for default model

[A9isha]

Description

Add an option for the test dataset to start at a specifiable index instead of always starting at 0 in train_rl.py.

Also, adding the MaxText default model to HF_IDS in src/maxtext/utils/globals.py so that it doesn't raise warning about tokenizer_path during vllm_config creation in train_rl.py

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Tests

CI and also ran locally on v5p-8

Checklist

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[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/trainers/post_train/rl/train_rl.py	0.00%	1 Missing ⚠️

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