More stable MBPP evaluation

[f14-bertolotti]

This PR improves the stability of the evaluation process for the MBPP dataset.

Motivation

There were two key issues affecting the robustness and reliability of the evaluation:

1. Regex performance:
   The regex pattern [r'(.*)\s*```.*'](https://github.com/open-compass/opencompass/blob/aa2b89b6f8b7c5448e47ed1aa3f12b04da1ff123/opencompass/datasets/mbpp.py#L327) occasionally hangs when processing predictions containing excessive trailing whitespace. This is likely due to catastrophic backtracking. To mitigate this, I replaced the standard re module with the regex module, which supports timeouts, and set a hard timeout of 10 seconds.

2. Multiprocessing issue:
   The following error was encountered:
   AttributeError: Can't pickle local object 'execution.<locals>._execution'
   This occurs because the ProcessPoolExecutor cannot pickle local (non-global) functions. The fix is to move the _execution function to the global scope so it can be properly serialized.

Modifications

1. Replaced re with regex and added a 10-second timeout to [regex.search](https://github.com/open-compass/opencompass/blob/aa2b89b6f8b7c5448e47ed1aa3f12b04da1ff123/opencompass/datasets/mbpp.py#L333).
2. Moved the _execution function [from here](https://github.com/open-compass/opencompass/blob/aa2b89b6f8b7c5448e47ed1aa3f12b04da1ff123/opencompass/datasets/mbpp.py#L403C1-L418C33) to the global scope to resolve the pickling issue.

Backward Compatibility

This change is not breaking. However, note that introducing a regex timeout may cause differences in evaluation results for cases where the timeout is triggered.