Keras Layer自定义（ layers.Layer）

来源：https://www.jianshu.com/p/6c34045216fb

实现一个简单层需要首先继承 layers.Layer 类即可,如下是官方网站上的例子:

rom keras import backend as K
from keras.engine.topology import Layer
import numpy as np

class MyLayer(Layer):

    def __init__(self, output_dim, **kwargs):
        self.output_dim = output_dim
        super(MyLayer, self).__init__(**kwargs)

    def build(self, input_shape):
        # Create a trainable weight variable for this layer.
        self.kernel = self.add_weight(name='kernel', 
                                      shape=(input_shape[1], self.output_dim),
                                      initializer='uniform',
                                      trainable=True)
        super(MyLayer, self).build(input_shape)  # Be sure to call this somewhere!

    def call(self, x):
        return K.dot(x, self.kernel)

    def compute_output_shape(self, input_shape):
        return (input_shape[0], self.output_dim)

如上所示, 其中有三个函数需要我们自己实现:

• build() 用来初始化定义weights, 这里可以用父类的self.add_weight() 函数来初始化数据, 该函数必须将 self.built 设置为True, 以保证该 Layer 已经成功 build , 通常如上所示, 使用 super(MyLayer, self).build(input_shape) 来完成
• call() 用来执行 Layer 的职能, 即当前 Layer 所有的计算过程均在该函数中完成
• compute_output_shape() 用来计算输出张量的 shape

正常DL都是一个forward, backword, update 三个流程,而在 keras 中对于单层 Layer 来说,通过将可训练的权应该在这里被加入列表`self.trainable_weights中。其他的属性还包括self.non_trainabe_weights（列表）和self.updates（需要更新的形如（tensor, new_tensor）的tuple的列表）。你可以参考BatchNormalization层的实现来学习如何使用上面两个属性。这个方法必须设置self.built = True，可通过调用super([layer],self).build()实现

loss 以及参数更新

详细查看了下 add_weight 函数实现如下（keras/engine/topology.py）：

def add_weight(self,
                   name,
                   shape,
                   dtype=None,
                   initializer=None,
                   regularizer=None,
                   trainable=True,
                   constraint=None):
        """Adds a weight variable to the layer.
        # Arguments
            name: String, the name for the weight variable.
            shape: The shape tuple of the weight.
            dtype: The dtype of the weight.
            initializer: An Initializer instance (callable).
            regularizer: An optional Regularizer instance.
            trainable: A boolean, whether the weight should
                be trained via backprop or not (assuming
                that the layer itself is also trainable).
            constraint: An optional Constraint instance.
        # Returns
            The created weight variable.
        """
        initializer = initializers.get(initializer)
        if dtype is None:
            dtype = K.floatx()
        weight = K.variable(initializer(shape),
                            dtype=dtype,
                            name=name,
                            constraint=constraint)
        if regularizer is not None:
            self.add_loss(regularizer(weight))
        if trainable:
            self._trainable_weights.append(weight)
        else:
            self._non_trainable_weights.append(weight)
        return weight

从上述代码来看通过 add_weight 创建的参数，通过 regularizer 函数来计算 loss, 如果 trainable 设置 True ，则该生成的 self._trainable_weights, 可以通过 regularizer 来构建 loss

具体训练过程参见: keras/engine/training.py