Fixes #1023: add custom processing logic for MetricGrouping

[colinzuo]

add another level of loop to dict fields in MetricGrouping instance

[colinzuo]

In LightevalTaskConfig:str, asdict(self) changed MetricGrouping to dict, so

adding an str method to the MetricGrouping

won't work
To stay consistent with the existing code, I added another level of loop for MetricGrouping, with no other changes