fix group args of mmlu and mmlu_pro (#2245)

lm_eval/tasks/mmlu/generative/_mmlu.yaml

@@ -26,7 +26,8 @@ task:
       - metric: acc
         weight_by_size: True
 aggregate_metric_list:
-  - metric: acc
+  - aggregation: mean
+    metric: exact_match
     weight_by_size: True
 metadata:
   version: 2

lm_eval/tasks/mmlu_pro/_mmlu_pro.yaml

+group: mmlu_pro
+task:
+  - mmlu_pro_biology
+  - mmlu_pro_business
+  - mmlu_pro_chemistry
+  - mmlu_pro_computer_science
+  - mmlu_pro_economics
+  - mmlu_pro_engineering
+  - mmlu_pro_health
+  - mmlu_pro_history
+  - mmlu_pro_law
+  - mmlu_pro_math
+  - mmlu_pro_other
+  - mmlu_pro_philosophy
+  - mmlu_pro_physics
+  - mmlu_pro_psychology
+aggregate_metric_list:
+  - aggregation: mean
+    metric: exact_match
+    weight_by_size: true
+    filter_list: custom-extract
+metadata:
+  version: 1.0
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