Adds MMLU CoT, gsm8k and arc_challenge for llama instruct

[anmarques]

This PR adds 3 tasks in the style used by Meta for Llama 3 models:

• mmlu cot
• arc_challenge
• gsm8k

The configs are very similar to the other llama3 tasks already supported.

Notes regarding arc_challenge:

• The original arc_challenge dataset contains 8 samples with less than 4 options. Meta filtered these samples out, and this PR does the same.
• A small number of samples use 1, 2, 3, 4 as labels. These are replaced by A, B, C, D like the rest in the doc preprocessing

Below are the results obtained with this PR for Llama-3.1-8B-Instruct:

Task	This PR	reference
MMLU CoT	72.63	73.0
ARC_Challenge	83.09	83.4
GSM8k	85.60	84.5

[baberabb]

This is great! I actually added arc_challenge before but missed your details. No wonder I couldn't reproduce the results.

Just need you to add the tasks in the Readme and should be good to merge. Can you also add your note as well? I think other users will find it quite helpful!

[anmarques]

This is great! I actually added arc_challenge before but missed your details. No wonder I couldn't reproduce the results.

Just need you to add the tasks in the Readme and should be good to merge. Can you also add your note as well? I think other users will find it quite helpful!

Thanks @baberabb. I just updated the README. Please let me know if you need anything else.

[baberabb]

Great! LGTM!