开源修改实现的null importance features select方法 及部分代码
附加了top1 top2 的主要解决方案
Xgboost-How to use “mae” as objective function? https://stackoverflow.com/questions/45006341/xgboost-how-to-use-mae-as-objective-function
GBDT自定义损失函数 https://cloud.tencent.com/developer/article/1357671