[Fix] Improve PubMedQA LLM-judge verdict robustness

[zzc152]

Motivation

While reproducing PubMedQA, I found two issues in opencompass/configs/datasets/PubMedQA/PubMedQA_llmjudge_gen_f00302.py, specifically in GRADER_TEMPLATE.

First, the template gives conflicting output instructions:

• A: CORRECT, B: INCORRECT
• "Just return the letters A or B"
• later, "Simply reply with either CORRECT, INCORRECT"

The judge therefore receives two incompatible output formats.

Second, PubMedQA is a multiple-choice task whose original answer labels are A/B/C. The same letters are visible in the question options, gold target, and candidate answer (for example, ANSWER: B), while A/B are also used as judge verdict labels. This label overlap can make the intended meaning of a generated A or B ambiguous.

For example, when the gold answer is B and the candidate prediction is also B, the correct judge verdict is A (CORRECT), but B is simultaneously the original task answer label. The same risk may exist in other multiple-choice benchmarks that reuse task-option labels as judge verdict labels; this PR is intentionally limited to the reproduced PubMedQA case.

Modification

• Use X for correct and Y for incorrect in the PubMedQA judge prompt.
• Configure generic_llmjudge_postprocess with true_tag='X' and false_tag='Y'.
• Document configurable judge verdict tags.
• Add a unit test for custom X/Y tags.

Validation

• Verified custom X/Y post-processing and PubMedQA config loading.
• Controlled experiments indicate that removing the conflicting instruction improves robustness; the label-overlap effect is model-dependent.

No changes are made to GenericLLMEvaluator or the generic parser.