LightGBM ranker函数使用什么评估指标

我在用 LGMRanker LightGBM 但不确定我应该使用什么评估指标。这是我的密码:

import lightgbm as lgb


gbm = lgb.LGBMRanker
gridParams = {
'learning_rate': [0.005,0.01,0.02],
'max_depth': [5,6,7],
'n_estimators': [100,200],
'num_leaves': [20,30,50]
}
lgb_grid = GridSearchCV(estimator = gbm, param_grid = gridParams, scoring = '??', cv = 3, verbose=2, n_jobs = -1)