如何在keras中减去频道均值？

import numpy as np
from keras.preprocessing.image import ImageDataGenerator

# 加载图像数据集
datagen = ImageDataGenerator()
dataset = datagen.flow_from_directory('path/to/dataset', target_size=(224, 224), batch_size=32)

# 计算频道均值
mean = np.mean(dataset, axis=(0, 1, 2))

# 减去频道均值
def preprocess_input(x):
    x -= mean
    return x

# 创建自定义的图像数据生成器
custom_datagen = ImageDataGenerator(preprocessing_function=preprocess_input)
custom_dataset = custom_datagen.flow_from_directory('path/to/dataset', target_size=(224, 224), batch_size=32)

# 使用减去频道均值后的图像数据进行训练或测试
model.fit(custom_dataset, ...)