我应该在哪里定义Keras中自定义激活函数的导数

from keras import backend as K
from keras.layers import Activation

class CustomActivation(Activation):
    def __init__(self, activation, **kwargs):
        super(CustomActivation, self).__init__(activation, **kwargs)
        self.__name__ = 'custom_activation'

def custom_activation(x):
    return K.sin(x)  # 自定义激活函数的计算逻辑

def custom_activation_derivative(x):
    return K.cos(x)  # 自定义激活函数的导数计算逻辑

# 在模型中使用自定义激活函数
model.add(Dense(64))
model.add(CustomActivation(custom_activation))

# 获取自定义激活函数的导数
x = K.placeholder(shape=(None, 64))
y = custom_activation(x)
grads = K.gradients(y, x)
custom_activation_derivative = K.function([x], grads)

# 使用自定义激活函数的导数
result = custom_activation_derivative([[1, 2, 3, 4]])
print(result)