XGBClassifier.feature_importances_函数，采用weight方式计算， * 'weight': the number of times a feature is used to split the data across all trees.一个特征用于在所有树上分割数据的次数。
* 'gain': the average gain across all splits the feature is used in.使用该特征的所有拆分的平均增益。
* 'cover': the average coverage across all splits the feature is used in.使用该特征的所有拆分的平均覆盖率。
* 'total_gain': the total gain across all splits the feature is used in.该特征在所有分割中使用的总增益。
* 'total_cover': the total coverage across all splits the feature is used in.使用该特征的所有拆分的总覆盖率。
