Adding rollout filtering to trainer

[LuanBrt]

Description

This PR adds rollout filtering to the Trainer, following the RAGEN approach. It helps prevent model collapse by retaining only the top N% of tasks with the highest outcome uncertainty.

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
• New tests have been added to cover the changes
• Tests have been run locally with python -m pytest tests/

Test Coverage

• Current coverage: 100%
• Coverage after changes: 100%

Checklist

• My code follows the style guidelines of this project
• 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

[CLAassistant]

All committers have signed the CLA.