为什么我的自定义lightgbm损耗没有减少?

我在lightgbm中实现了一个自定义平均误差(MAE)损失。梯度不是零,但损耗保持不变。怎么可能?

我的实施:

def abs_obj(preds, dtrain):
    y_true = dtrain.get_label()
    a = preds - y_true
    grad = np.sign(a)
    hess = np.zeros(len(a))
    return grad, hess

def abs_eval(preds, dtrain):
    y_true = dtrain.get_label()
    loss = np.abs(preds - y_true).sum()
    return "error", loss, False

一个最小的可重复性示例:损失保持不变。

dtrain = pd.DataFrame({'x':np.random.rand(100),
                      'y':np.random.rand(100)})
ytrain = dtrain.x + 2 * dtrain.y
dval = dtrain
yval = ytrain
lgb_train = lgb.Dataset(dtrain, ytrain)
lgb_valid = lgb.Dataset(dval, yval)
params = {'objective':None,
         'learning_rate':30,
         'num_leaves':33}
clf = lgb.train(params,
               lgb_train,
               valid_sets=[lgb_valid],
               num_boost_round=10,
               verbose_eval=1,
               fobj=abs_obj,
               feval=abs_eval)