Adds MMLU CoT, gsm8k and arc_challenge for llama instruct (#2829)

* llama-style MMLU CoT

* Refactor MMLU CoT template YAML to simplify 'until' structure

* Add GSM8K task configuration for LLaMA3 with few-shot examples

* Fix missing newline at end of MMLU CoT YAML file

* Add ARC-Challenge task configuration and processing utility

* Add additional MMLU and ARC-Challenge task variants to README

* Update README with notes on arc_challenge_llama dataset preprocessing

lm_eval/tasks/llama3/instruct/mmlu_cot/mmlu_professional_psychology.yaml

+"dataset_name": "professional_psychology"
+"description": ""
+"include": "_mmlu_cot_llama_template_yaml"
+"tag": "mmlu_cot_llama_social_sciences_tasks"
+"task": "mmlu_cot_llama_professional_psychology"
+"task_alias": "professional psychology"

lm_eval/tasks/llama3/instruct/mmlu_cot/mmlu_public_relations.yaml

+"dataset_name": "public_relations"
+"description": ""
+"include": "_mmlu_cot_llama_template_yaml"
+"tag": "mmlu_cot_llama_social_sciences_tasks"
+"task": "mmlu_cot_llama_public_relations"
+"task_alias": "public relations"

lm_eval/tasks/llama3/instruct/mmlu_cot/mmlu_security_studies.yaml

+"dataset_name": "security_studies"
+"description": ""
+"include": "_mmlu_cot_llama_template_yaml"
+"tag": "mmlu_cot_llama_social_sciences_tasks"
+"task": "mmlu_cot_llama_security_studies"
+"task_alias": "security studies"

lm_eval/tasks/llama3/instruct/mmlu_cot/mmlu_sociology.yaml

+"dataset_name": "sociology"
+"description": ""
+"include": "_mmlu_cot_llama_template_yaml"
+"tag": "mmlu_cot_llama_social_sciences_tasks"
+"task": "mmlu_cot_llama_sociology"
+"task_alias": "sociology"

lm_eval/tasks/llama3/instruct/mmlu_cot/mmlu_us_foreign_policy.yaml

+"dataset_name": "us_foreign_policy"
+"description": ""
+"include": "_mmlu_cot_llama_template_yaml"
+"tag": "mmlu_cot_llama_social_sciences_tasks"
+"task": "mmlu_cot_llama_us_foreign_policy"
+"task_alias": "us foreign policy"

lm_eval/tasks/llama3/instruct/mmlu_cot/mmlu_virology.yaml

+"dataset_name": "virology"
+"description": ""
+"include": "_mmlu_cot_llama_template_yaml"
+"tag": "mmlu_cot_llama_other_tasks"
+"task": "mmlu_cot_llama_virology"
+"task_alias": "virology"

lm_eval/tasks/llama3/instruct/mmlu_cot/mmlu_world_religions.yaml

+"dataset_name": "world_religions"
+"description": ""
+"include": "_mmlu_cot_llama_template_yaml"
+"tag": "mmlu_cot_llama_humanities_tasks"
+"task": "mmlu_cot_llama_world_religions"
+"task_alias": "world religions"