fix boxed parsing for mme_realworld

[bkal01]

samples in DatBench from mme_realworld have prompt_format as follows:

{
"prefix": "",
"suffix": "\nA. Two of the construction vehicles are parked, and one is moving.\nB. One of the construction vehicles is moving, and one is parked.\nC. Three construction vehicles are parked.\nD. Many construction vehicles are parked.\nE. The image does not feature the object.\nSelect the correct option.\nProvide only \\boxed{<LETTER>} (uppercase A/B/C/D/E) on the answer line.\nAnswer:"
}

so we expect the model to output something like:

The correct answer is D.

\boxed{D}

the previous regex pattern doesn't parse this correctly because it is over-escaped:

the updated pattern (copied from here) does:

there's a fallback in mme_realworld.py to look for the first standalone letter. this is problematic in some cases where model output contains some additional text:

This is a question about construction vehicles. The correct answer is D.

\boxed{D}

the old regex would parse "A" as a standalone letter, despite the model outputting "D". the updated regex fixes this.