log lr hyperparams; add exact match reward

[andytwigg]

Description

inject LR as hyperparam to let tunix log LR
add exact match reward

Tests

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commands to reproduce.

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[codecov]

Codecov Report

❌ Patch coverage is 7.14286% with 13 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/trainers/post_train/rl/utils_rl.py	0.00%	13 Missing ⚠️

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[richjames0]

Thanks Andy. This also includes a bunch of configs so maybe just update the PR title to reflect that