feat: Add mmlu-redux and it's spanish transaltion as generative task definitions

[luiscosio]

This PR adds generative task definitions for two MMLU-Redux datasets:

• Original MMLU-Redux-2.0 (english) from edinburgh-dawg/mmlu-redux-2.0
• Spanish translation of MMLU-Redux-2.0

The task definitions follow the same structure and evaluation metrics as existing MMLU tasks, using exact_match for scoring with weight_by_size enabled. Both datasets are organized into 4 main groups:

• STEM
• Other
• Social Sciences
• Humanities

Each group maintains consistent evaluation metrics and aggregation methods across both language versions.

Changes include:

• Added task definitions for generative format evaluation
• Consistent group structure between English and Spanish versions
• Maintained weight_by_size true for all metrics
• Version 3 metadata tag for compatibility

This enhancement allows for direct comparison of model performance between English and Spanish versions of MMLU-Redux in a generative setting.

[baberabb]

Hi! Thanks for the PR. Just some minor issues:

1. test is failing as it can't find one of the subtask configs on HF hub (probably a typo).
2. Could you add the readme from template/new_yaml_task, and also add an entry in lm_eval/tasks/README.md