[Enhancement] Add humaneval postprocessor for GPT models & eval config for GPT4, enhance the original humaneval postprocessor

[gaotongxiao]

Enhance humaneval postprocessor so that it handles more corner cases. This PR needs to be tested on other models thoroughly before being merged.

new case 1

Some models may wrap the result in the code block annotated with the language name.

```python
# code to be extracted
```

new case 2

Some models may first repeat the question before generating the completion code.

# question's body
import xxx

def xxx():
   # model's generation

re-requesting review