[WIP] add balanced_accuracy_score metric #3506

It refers to the issue https://github.com/scikit-learn/scikit-learn/issues/3506

- It passes all the common tests for metrics and regression metrics, but I still need to implement specific tests.
- It might be a duplicate of the work done by @lazywei at https://github.com/scikit-learn/scikit-learn/pull/3511 , so I will compare the two versions
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