mmlu-pro: update yaml content in line with mmlu

lm_eval/tasks/mmlu_pro/generative/mmlu_pro_biology.yaml

 "dataset_name": "biology"
 "description": "The following are multiple choice questions (with answers) about biology.\n\
   \n"
-"group": "mmlu_pro_stem_generative"
-"group_alias": "stem"
+"tag": "mmlu_pro_stem_generative"
 "include": "_default_template_yaml"
 "task": "mmlu_pro_biology_generative"
 "task_alias": "biology"

lm_eval/tasks/mmlu_pro/generative/mmlu_pro_psychology.yaml

 "dataset_name": "psychology"
 "description": "The following are multiple choice questions (with answers) about psychology.\n\
   \n"
-"group": "mmlu_pro_social_sciences_generative"
-"group_alias": "social_sciences"
+"tag": "mmlu_pro_social_sciences_generative"
 "include": "_default_template_yaml"
 "task": "mmlu_pro_psychology_generative"
 "task_alias": "psychology"