fix some bugs of mmlu

[eyuansu62]

We find the following bugs of mmlu:

1. The doc_to_text function in mmlu_flan_cot_fewshot is incorrect, causing the few-shot data not aligning with the pre-defined examples specified in the fewshot_config for each subset. For example, now we get:

Q: Statement 1 | Every element of a group generates a cyclic subgroup of the group. Statement 2 | The symmetric group S_10 has 10 elements.
(A) True, True (B) False, False (C) True, False (D) False, True
A: Let's think step by step. (C)

But the right one should be:

Q: Statement 1 | Every element of a group generates a cyclic subgroup of the group. Statement 2 | The symmetric group S_10 has 10 elements.
(A) True, True (B) False, False (C) True, False (D) False, True
A: Let's think step by step. A cyclic group is a group that is generated by a single element. Hence a subgroup generated by a single element of a group is cyclic and Statement 1 is True. The answer is (C).

2. Since not all subsets have 5 cot-examples in "_cot_prompts.json", we set the default num_fewshot to 4.
3. We modify the filter setting of mmlu_flan_n_shot to solve the related issue https://github.com/EleutherAI/lm-evaluation-harness/issues/2094#issuecomment-2260772114.

[baberabb]

Thanks for the PR. Looks reasonable to me but I want @lintangsutawika to have a look as well.