Add MMLU multiple choice 5-shot

[yulinggu-cs]

Added MMLU multiple choice (A/B/C/D) 5-shot variant. This follows the prompt formatting in Eleuther AI Harness.
This will show up in wandb with "len_norm" tag but since the predicted continuation is just a single token, it is the same as acc.

List of tasks available to use in training config:

    - label: mmlu_stem_mc_5shot
      type: downstream
    - label: mmlu_humanities_mc_5shot
      type: downstream
    - label: mmlu_social_sciences_mc_5shot
      type: downstream
    - label: mmlu_other_mc_5shot
      type: downstream

This will run eval on the validation partition, if running on the test partition (10X larger) is desired, add _test to the label name e.g., mmlu_stem_mc_5shot_test

Example:

doc_text: The following are multiple choice questions (with answers) about global facts:

Which of the following pairs of statements are both true (as of 2019)?
A. People tend to be optimistic about their own future and the future of their nation or the world.
B. People tend to be optimistic about their own future but pessimistic about the future of their nation or the world.
C. People tend to be pessimistic about their own future but optimistic about the future of their nation or the world.
D. People tend to be pessimistic about their own future and the future of their nation or the world.
Answer: B

As of 2019, about what percentage of Americans agree that the state is run for the benefit of all the people?
A. 31%
B. 46%
C. 61%
D. 76%
Answer: B

As of 2015, since 1990 forests have ____ in Europe and have ____ in Africa and the Americas.
A. increased, increased
B. increased, decreased
C. decreased, increased
D. decreased, decreased
Answer: B

As of 2019, about what percentage of Russians say it is very important to have free media in our country without government/state censorship?
A. 38%
B. 53%
C. 68%
D. 83%
Answer: A

As of 2017, how many of the world’s 1-year-old children today have been vaccinated against some disease? *
A. 80%
B. 60%
C. 40%
D. 20%
Answer: A

As of 2020, about what percentage of the population in Sub-Saharan Africa, is living on less than $1.90?
A. 10%
B. 20%
C. 40%
D. 80%
Answer:
continuations: [' A', ' B', ' C', ' D']

[OyvindTafjord]

This PR was passing all tests before merging in some small recent changes (771c828), not sure what's going on?

[epwalsh]

Thanks @yulinggu-cs, let's ignore the CI error since it's unrelated.