Upgrade default math pred_postprocessor

[xu-song]

Upgrade default math pred_postprocessor from math_postprocess to math_postprocess_v2

Motivation

As mentioned in document, the default pred_postprocessor is math_postprocess_v2 in both math_4shot_base_gen_db136b and math_0shot_gen_393424.

math_postprocess_v2 is more robust than math_postprocess.

opencompass/opencompass/datasets/math.py

Lines 158 to 161 in 1f9f728

	def math_postprocess(text: str) -> str:
	for maybe_ans in text.split('.'):
	if 'final answer' in maybe_ans.lower():

[liushz]

LGTM