如何获取xgb.DMatrix的索引，为feval函数添加信息？

import xgboost as xgb

# 加载数据集
data = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
labels = [1, 0, 1]
dtrain = xgb.DMatrix(data, label=labels)

def custom_eval(preds, dtrain):
    labels = dtrain.get_label()  # 获取训练集的标签
    # 在这里添加你想要的额外信息
    # 例如，计算准确率
    accuracy = sum(labels == (preds > 0.5)) / len(labels)
    return 'accuracy', accuracy

params = {'objective': 'binary:logistic', 'eval_metric': 'logloss'}
num_rounds = 10

# 训练模型并使用自定义的评估函数
xgb.train(params, dtrain, num_rounds, evals=[(dtrain, 'train')], feval=custom_eval)