如何将验证数据整合到Keras的datagen.flow中？

from keras.preprocessing.image import ImageDataGenerator

# 创建训练数据生成器
train_datagen = ImageDataGenerator(
    rotation_range=20,
    width_shift_range=0.2,
    height_shift_range=0.2,
    shear_range=0.2,
    zoom_range=0.2,
    horizontal_flip=True
)

# 创建验证数据生成器
val_datagen = ImageDataGenerator()

# 从文件夹中读取训练数据和验证数据
train_generator = train_datagen.flow_from_directory(
    'train',
    target_size=(224, 224),
    batch_size=32,
    class_mode='categorical'
)

val_generator = val_datagen.flow_from_directory(
    'validation',
    target_size=(224, 224),
    batch_size=32,
    class_mode='categorical'
)

model.fit_generator(
    train_generator,
    steps_per_epoch=len(train_generator),
    epochs=10,
    validation_data=val_generator,
    validation_steps=len(val_generator)
)