Group agg rework

[lintangsutawika]

1. By default, group will not feature the aggregate scores of their subtasks.
2. To show an aggregate, a group_config will need to be defined in the yaml that consists of aggregate_metric (True/False, default False) and weight_by_size (True/False, default False).
3. Use task_id in ConfigurableGroup and ConfigurableTask to be used as identifier in lieu of task name/group name.

[lintangsutawika]

@haileyschoelkopf I've only added the group_config to MMLU tasks and flan_held_in. Let me know what other benchmarks that would need the aggregation to be added back in.

[haileyschoelkopf]

aggregate fn being defined explicitly is good, I think we also need to try to be careful of how tasks with (multiple) filters interact with groups.

Currently, if two tasks have filters with different names, a group can't aggregate those correctly. We may want to have groups, for each metric they'll report, expressly define, for each subtask, what metric and what filter to include in the aggregation.