Fix grpo recipe

[salygin]

Context

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• add a new feature
• fix a bug
• update tests and/or documentation
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Changelog

What are the changes made in this PR?

• Configurable reward functions #2698

Test plan

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UX

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• I did not change any public API
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[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchtune/2893

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⏳ No Failures, 2 Pending

As of commit ffd3650 with merge base 4249bd2 ():
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[krammnic]

LGTM. It is proper way to fix this, before I will merge #2876