Fixed a bug that in MMLU-Pro utils.py that throw index error if one choice was removed

[sleepingcat4]

I had recently been experimenting with eval datasets and found in MMLU-Pro, if I removed a choice despite the utils.py code being concrete written for edge cases where a choice can be removed, it was still throwing an index-error. It didn't make sense. I had talked with baber and he had mentioned to work with a debugger and fix it. But, I was able to fix it and adding this modification.

Now the code works both with default 10 choices and modified number of choices (only for cases where choices are removed)

[CLAassistant]

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