Show intermediate reward in vf-eval

[mikasenghaas]

Description

Show intermediate reward in vf-eval via tqdm postfix as groups finish

 uv run vf-eval gsm8k

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

---

Note

Adds a rolling average reward display to the tqdm progress bar during generation/evaluation.

• Initializes tqdm with postfix and tracks reward_sum/reward_count from completed group states
• Updates progress with pbar.set_postfix(reward=...) to show the current average reward as groups finish

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