Почему не начинается процесс обучения keras?

model = models.Sequential()
model.add(layers.Conv2D(32, (3, 3), activation="relu", input_shape=(200, 200, 3)))
model.add(layers.MaxPooling2D((2, 2)))
model.add(layers.Conv2D(64, (3, 3), activation="relu"))
model.add(layers.MaxPooling2D((2, 2)))
model.add(layers.Conv2D(128, (3, 3), activation="relu"))
model.add(layers.MaxPooling2D((2, 2)))
model.add(layers.Conv2D(128, (3, 3), activation="relu"))
model.add(layers.MaxPooling2D((2, 2)))
model.add(layers.Flatten())
model.add(layers.Dense(512, activation="relu"))
model.add(layers.Dense(4))
model.compile(optimizer="rmsprop", loss="mse", metrics=["mae"])

train_dir = "path/to/faces/1"
val_dir = "path/to/faces/2"
train_gen = ImageDataGenerator(rescale=1./255)
val_gen = ImageDataGenerator(rescale=1./255)
train_generator = train_gen.flow_from_directory(train_dir, target_size=(200, 200), batch_size=50, class_mode="categorical")
validation_generator = val_gen.flow_from_directory(val_dir, target_size=(200, 200), batch_size=50, class_mode="categorical")
model.fit_generator(train_generator, steps_per_epoch=200, epochs=3, validation_data=validation_generator, validation_steps=100)