1 构建一个简单的网络层
我们可以通过继承tf.keras.layer.Layer,实现一个自定义的网络层。
In [1]:
from __future__ import absolute_import, division, print_function
import tensorflow as tf
tf.keras.backend.clear_session()
import tensorflow.keras as keras
import tensorflow.keras.layers as layers
In [2]:
# 定义网络层就是：设置网络权重和输出到输入的计算过程
class MyLayer(layers.Layer):
def __init__(self, input_dim=32, unit=32):
super(MyLayer, self).__init__()
w_init = tf.random_normal_initializer()
# 权重变量
self.weight = tf.Variable(initial_value=w_init(
shape=(input_dim, unit), dtype=tf.float32), trainable=True)
b_init = tf.zeros_initializer()
# 偏置变量
self.bias = tf.Variable(initial_value=b_init(
shape=(unit,), dtype=tf.float32), trainable=True)
def call(self, inputs):
# 全连接网络
return tf.matmul(inputs, self.weight) + self.bias
x = tf.ones((3,5))
my_layer = MyLayer(5, 4)
out = my_layer(x)
print(out)
按上面构建网络层，图层会自动跟踪权重w和b，当然我们也可以直接用add_weight的方法构建权重
In [3]:
class MyLayer(layers.Layer):
def __init__(self, input_dim=32, unit=32):
super(MyLayer, self).__init__()
# 使用add_weight添加网络变量，使其可追踪
self.weight = self.add_weight(shape=(input_dim, unit),
initializer=keras.initializers.RandomNormal(),
trainable=True)
self.bias = self.add_weight(shape=(unit,),
initializer=keras.initializers.Zeros(),
trainable=True)
def call(self, inputs):
return tf.matmul(inputs, self.weight) + self.bias
x = tf.ones((3,5))
my_layer = MyLayer(5, 4)
out = my_layer(x)
print(out)
也可以设置不可训练的权重
In [4]:
class AddLayer(layers.Layer):
def __init__(self, input_dim=32):
super(AddLayer, self).__init__()
# 只存储，不训练的变量
self.sum = self.add_weight(shape=(input_dim,),
initializer=keras.initializers.Zeros(),
trainable=False)
def call(self, inputs):
self.sum.assign_add(tf.reduce_sum(inputs, axis=0))
return self.sum
x = tf.ones((3,3))
my_layer = AddLayer(3)
out = my_layer(x)
print(out.numpy())
out = my_layer(x)
print(out.numpy())
print('weight:', my_layer.weights)
print('non-trainable weight:', my_layer.non_trainable_weights)
print('trainable weight:', my_layer.trainable_weights)
当定义网络时不知道网络的维度是可以重写build()函数，用获得的shape构建网络
In [5]:
class MyLayer(layers.Layer):
def __init__(self, unit=32):
super(MyLayer, self).__init__()
self.unit = unit
def build(self, input_shape):
# 在build时获取input_shape
self.weight = self.add_weight(shape=(input_shape[-1], self.unit),
initializer=keras.initializers.RandomNormal(),
trainable=True)
self.bias = self.add_weight(shape=(self.unit,),
initializer=keras.initializers.Zeros(),
trainable=True)
def call(self, inputs):
return tf.matmul(inputs, self.weight) + self.bias
my_layer = MyLayer(3)
x = tf.ones((3,5))
out = my_layer(x)
print(out)
my_layer = MyLayer(3)
x = tf.ones((2,2))
out = my_layer(x)
print(out)
2 使用子层递归构建网络层
可以在自定义网络层中调用其他自定义网络层
In [6]:
class MyBlock(layers.Layer):
def __init__(self):
super(MyBlock, self).__init__()
# 其他自定义网络层
self.layer1 = MyLayer(32)
self.layer2 = MyLayer(16)
self.layer3 = MyLayer(2)
def call(self, inputs):
h1 = self.layer1(inputs)
h1 = tf.nn.relu(h1)
h2 = self.layer2(h1)
h2 = tf.nn.relu(h2)
return self.layer3(h2)
my_block = MyBlock()
print('trainable weights:', len(my_block.trainable_weights))
y = my_block(tf.ones(shape=(3, 64)))
# 构建网络在build()里面，所以执行了才有网络
print('trainable weights:', len(my_block.trainable_weights))
可以通过构建网络层的方法来收集loss，并可以递归调用。
In [7]:
class LossLayer(layers.Layer):
def __init__(self, rate=1e-2):
super(LossLayer, self).__init__()
self.rate = rate
def call(self, inputs):
# 添加loss
self.add_loss(self.rate * tf.reduce_sum(inputs))
return inputs
class OutLayer(layers.Layer):
def __init__(self):
super(OutLayer, self).__init__()
self.loss_fun=LossLayer(1e-2)
def call(self, inputs):
# 就一个loss层
return self.loss_fun(inputs)
my_layer = OutLayer()
print(len(my_layer.losses)) # 还未call
y = my_layer(tf.zeros(1,1))
print(len(my_layer.losses)) # 执行call之后
y = my_layer(tf.zeros(1,1))
print(len(my_layer.losses)) # call之前会重新置0
如果中间调用了keras网络层，里面的正则化loss也会被加入进来
In [8]:
class OuterLayer(layers.Layer):
def __init__(self):
super(OuterLayer, self).__init__()
# 子层中正则化loss也会添加到总的loss中
self.dense = layers.Dense(32, kernel_regularizer=tf.keras.regularizers.l2(1e-3))
def call(self, inputs):
return self.dense(inputs)
my_layer = OuterLayer()
y = my_layer(tf.zeros((1,1)))
print(my_layer.losses)
print(my_layer.weights)
3 其他网络层配置
3.1 使自己的网络层可以序列化
In [9]:
class Linear(layers.Layer):
def __init__(self, units=32, **kwargs):
super(Linear, self).__init__(**kwargs)
self.units = units
def build(self, input_shape):
self.w = self.add_weight(shape=(input_shape[-1], self.units),
initializer='random_normal',
trainable=True)
self.b = self.add_weight(shape=(self.units,),
initializer='random_normal',
trainable=True)
def call(self, inputs):
return tf.matmul(inputs, self.w) + self.b
def get_config(self):
# 获取网络配置，用于实现序列化
config = super(Linear, self).get_config()
config.update({'units':self.units})
return config
layer = Linear(125)
config = layer.get_config()
print(config)
# 从配置中构建网络，（已知网络结构，不知超参的情况）
new_layer = Linear.from_config(config)
如果在反序列化中(从配置中构建网络)需要更大的灵活性，可以重写from_config方法。
In [10]:
def from_config(cls, config):
return cls(**config)
3.2 配置训练时特有参数
有一些网络层， 如BatchNormalization层和Dropout层，在训练和推理中具有不同的行为，对于此类层，则需要在方法中使用train等参数进行控制。
In [11]:
class MyDropout(layers.Layer):
def __init__(self, rate, **kwargs):
super(MyDropout, self).__init__(**kwargs)
self.rate = rate
def call(self, inputs, training=None):
return tf.cond(training,
lambda: tf.nn.dropout(inputs, rate=self.rate),
lambda: inputs)
4 构建自己的模型
通常，我们使用Layer类来定义内部计算块，并使用Model类来定义外部模型 - 即要训练的对象。
Model类与Layer的区别：
- 它对外开放内置的训练，评估和预测函数（model.fit(),model.evaluate(),model.predict()）。
- 它通过model.layers属性开放其内部网络层列表。
- 它对外开放保存和序列化API。
4.1 自定义模型
下面通过构建一个变分自编码器（VAE），来介绍如何构建自己的网络， 并使用内置的函数进行训练。
In [12]:
# 采样网络
class Sampling(layers.Layer):
def call(self, inputs):
z_mean, z_log_var = inputs
batch = tf.shape(z_mean)[0]
dim = tf.shape(z_mean)[1]
epsilon = tf.keras.backend.random_normal(shape=(batch, dim))
return z_mean + tf.exp(0.5*z_log_var) * epsilon
# 编码器
class Encoder(layers.Layer):
def __init__(self, latent_dim=32,
intermediate_dim=64, name='encoder', **kwargs):
super(Encoder, self).__init__(name=name, **kwargs)
self.dense_proj = layers.Dense(intermediate_dim, activation='relu')
self.dense_mean = layers.Dense(latent_dim)
self.dense_log_var = layers.Dense(latent_dim)
self.sampling = Sampling()
def call(self, inputs):
h1 = self.dense_proj(inputs)
# 获取z_mean和z_log_var
z_mean = self.dense_mean(h1)
z_log_var = self.dense_log_var(h1)
# 进行采样
z = self.sampling((z_mean, z_log_var))
return z_mean, z_log_var, z
# 解码器
class Decoder(layers.Layer):
def __init__(self, original_dim,
intermediate_dim=64, name='decoder', **kwargs):
super(Decoder, self).__init__(name=name