feat: display running average of metrics during `vf-eval`

[anakin87]

Description

Fixes #416.

When running vf-eval, the average reward and completion length are now displayed in the progress bar as rollouts complete. To do this, I added a tqdm_gather_with_metrics utility function that aggregates metrics during async task execution. This function is used in a_generate to show running averages.

(@mikasenghaas take a look)

Example output

https://github.com/user-attachments/assets/721bc395-8491-4c8d-b330-16fd34f8617f

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] 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

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

Checklist

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

Additional Notes

[FedericoBonel]

We probably should support overriding the viewport controls as well (zoom and viewport size buttons). There are members of the community requesting this on Discord.