如何在训练 tensorflow.keras 期间替换损失函数

Question

我想在训练期间替换与我的神经网络相关的损失函数，这是网络：

model = tensorflow.keras.models.Sequential()
        model.add(tensorflow.keras.layers.Conv2D(32, kernel_size=(3, 3), activation="relu", input_shape=input_shape))
        model.add(tensorflow.keras.layers.Conv2D(64, (3, 3), activation="relu"))
        model.add(tensorflow.keras.layers.MaxPooling2D(pool_size=(2, 2)))
        model.add(tensorflow.keras.layers.Dropout(0.25))
        model.add(tensorflow.keras.layers.Flatten())
        model.add(tensorflow.keras.layers.Dense(128, activation="relu"))
        model.add(tensorflow.keras.layers.Dropout(0.5))
        model.add(tensorflow.keras.layers.Dense(output_classes, activation="softmax"))
        model.compile(loss=tensorflow.keras.losses.categorical_crossentropy, optimizer=tensorflow.keras.optimizers.Adam(0.001), metrics=['accuracy'])
        history = model.fit(x_train, y_train, batch_size=128, epochs=5, validation_data=(x_test, y_test))

所以现在我想用另一个更改tensorflow.keras.losses.categorical_crossentropy，所以我做了这个：

model.compile(loss=tensorflow.keras.losses.mse, optimizer=tensorflow.keras.optimizers.Adam(0.001), metrics=['accuracy'])
    history = model.fit(x_improve, y_improve, epochs=1, validation_data=(x_test, y_test)) #FIXME bug during training

但我有这个错误：

ValueError: No gradients provided for any variable: ['conv2d/kernel:0', 'conv2d/bias:0', 'conv2d_1/kernel:0', 'conv2d_1/bias:0', 'dense/kernel:0', 'dense/bias:0', 'dense_1/kernel:0', 'dense_1/bias:0'].

为什么？我该如何解决？还有其他方法可以改变损失函数吗？

谢谢

Answer 1

我目前正在使用 Tensorflow 和 Keras 开发 google colab，但每次我重新编译这样的模型时，我都无法重新编译保持权重的模型：

with strategy.scope():
  model = hd_unet_model(INPUT_SIZE)
  model.compile(optimizer=Adam(lr=0.01), 
                loss=tf.keras.losses.MeanSquaredError() ,
                metrics=[tf.keras.metrics.MeanSquaredError()]) 

权重被重置。 所以我找到了另一个解决方案，你需要做的就是：

1. 获取具有所需权重的模型（加载或其他）
2. 这样获取模型的权重：

weights = model.get_weights()

1. 重新编译模型（改变损失函数）
2. 再次设置重新编译模型的权重，如下所示：

model.set_weights(weights)

5. 启动培训

我测试了这个方法，它似乎有效。

所以要改变训练中的损失，你可以：

1. 用第一个损失编译。
2. 第一次损失的火车。
3. 保存权重。
4. 用第二个损失重新编译。
5. 加载砝码。
6. 第二次损失训练。

Answer 2

所以，我会给出一个直截了当的答案：如果你想玩这种游戏，请切换到 pytorch。由于在 pytorch 中定义了训练和评估函数，因此只需一个 if 语句即可从损失函数切换到另一个损失函数。

另外，我在你的代码中看到你想从 cross_entropy 切换到 mean_square_error，前者适合分类，后者适合回归，所以这不是你可以做的，在下面的代码中我从 mean 切换平方误差到均方对数误差，都是适合回归的损失。

尽管其他答案为您的问题提供了解决方案（请参阅change-loss-function-dynamically-during-training），但尚不清楚您是否可以信任结果。有些人发现，即使使用自定义函数，有时 Keras 也会在第一次损失时继续训练。

解决方案：

我的解决方案基于 train_on_batch，它允许我们在 for 循环中训练模型，因此只要我们希望使用新的损失函数重新编译模型时就停止训练它。请注意，重新编译模型不会重置权重（请参阅：Does recompiling a model re-initialize the weights?）。

数据集可以在这里找到Boston housing dataset

# Regression Example With Boston Dataset: Standardized and Larger
from pandas import read_csv
from keras.models import Sequential
from keras.layers import Dense
from sklearn.model_selection import train_test_split
from keras.losses import mean_squared_error, mean_squared_logarithmic_error
from matplotlib import pyplot
import matplotlib.pyplot as plt

# load dataset
dataframe = read_csv("housing.csv", delim_whitespace=True, header=None)
dataset = dataframe.values

# split into input (X) and output (Y) variables
X = dataset[:,0:13]
y = dataset[:,13]

trainX, testX, trainy, testy = train_test_split(X, y, test_size=0.33, random_state=42)

# create model
model = Sequential()
model.add(Dense(13, input_dim=13, kernel_initializer='normal', activation='relu'))
model.add(Dense(6, kernel_initializer='normal', activation='relu'))
model.add(Dense(1, kernel_initializer='normal'))

batch_size = 25

# have to define manually a dict to store all epochs scores 
history = {}
history['history'] = {}
history['history']['loss'] = []
history['history']['mean_squared_error'] = []
history['history']['mean_squared_logarithmic_error'] = []
history['history']['val_loss'] = []
history['history']['val_mean_squared_error'] = []
history['history']['val_mean_squared_logarithmic_error'] = []

# first compiling with mse
model.compile(loss='mean_squared_error', optimizer='adam', metrics=[mean_squared_error, mean_squared_logarithmic_error])

# define number of iterations in training and test
train_iter = round(trainX.shape[0]/batch_size)
test_iter = round(testX.shape[0]/batch_size)

for epoch in range(2):
    
    # train iterations 
    loss, mse, msle = 0, 0, 0
    for i in range(train_iter):
        
        start = i*batch_size
        end = i*batch_size + batch_size
        batchX = trainX[start:end,]
        batchy = trainy[start:end,]
        
        loss_, mse_, msle_ = model.train_on_batch(batchX,batchy)
                
        loss += loss_
        mse += mse_
        msle += msle_
    
    history['history']['loss'].append(loss/train_iter)
    history['history']['mean_squared_error'].append(mse/train_iter)
    history['history']['mean_squared_logarithmic_error'].append(msle/train_iter)
    
    # test iterations 
    val_loss, val_mse, val_msle = 0, 0, 0
    for i in range(test_iter):
        
        start = i*batch_size
        end = i*batch_size + batch_size
        batchX = testX[start:end,]
        batchy = testy[start:end,]
        
        val_loss_, val_mse_, val_msle_ = model.test_on_batch(batchX,batchy)
        
        val_loss += val_loss_
        val_mse += val_mse_
        val_msle += msle_
        
    history['history']['val_loss'].append(val_loss/test_iter)
    history['history']['val_mean_squared_error'].append(val_mse/test_iter)
    history['history']['val_mean_squared_logarithmic_error'].append(val_msle/test_iter)
    
# recompiling the model with new loss
model.compile(loss='mean_squared_logarithmic_error', optimizer='adam', metrics=[mean_squared_error, mean_squared_logarithmic_error])

for epoch in range(2):
    
    # train iterations 
    loss, mse, msle = 0, 0, 0
    for i in range(train_iter):
        
        start = i*batch_size
        end = i*batch_size + batch_size
        batchX = trainX[start:end,]
        batchy = trainy[start:end,]
    
        loss_, mse_, msle_ = model.train_on_batch(batchX,batchy)
        
        loss += loss_
        mse += mse_
        msle += msle_
        
    history['history']['loss'].append(loss/train_iter)
    history['history']['mean_squared_error'].append(mse/train_iter)
    history['history']['mean_squared_logarithmic_error'].append(msle/train_iter)
     
    # test iterations 
    val_loss, val_mse, val_msle = 0, 0, 0
    for i in range(test_iter):
        
        start = i*batch_size
        end = i*batch_size + batch_size
        batchX = testX[start:end,]
        batchy = testy[start:end,]
        
        val_loss_, val_mse_, val_msle_ = model.test_on_batch(batchX,batchy)
        
        val_loss += val_loss_
        val_mse += val_mse_
        val_msle += msle_
        
    history['history']['val_loss'].append(val_loss/test_iter)
    history['history']['val_mean_squared_error'].append(val_mse/test_iter)
    history['history']['val_mean_squared_logarithmic_error'].append(val_msle/test_iter)
    
# Some plots to check what is going on   
# loss function 
pyplot.subplot(311)
pyplot.title('Loss')
pyplot.plot(history['history']['loss'], label='train')
pyplot.plot(history['history']['val_loss'], label='test')
pyplot.legend()

# Only mean squared error 
pyplot.subplot(312)
pyplot.title('Mean Squared Error')
pyplot.plot(history['history']['mean_squared_error'], label='train')
pyplot.plot(history['history']['val_mean_squared_error'], label='test')
pyplot.legend()

# Only mean squared logarithmic error 
pyplot.subplot(313)
pyplot.title('Mean Squared Logarithmic Error')
pyplot.plot(history['history']['mean_squared_logarithmic_error'], label='train')
pyplot.plot(history['history']['val_mean_squared_logarithmic_error'], label='test')
pyplot.legend()
plt.tight_layout()
pyplot.show()

结果图确认损失函数在第二个 epoch 之后发生了变化：

损失函数的下降是由于模型从正常均方误差转换为对数误差，后者的值要低得多。打印分数也证明使用的损失确实发生了变化：

print(history['history']['loss'])
[599.5209197998047, 570.4041115897043, 3.8622902120862688, 2.1578191178185597]
print(history['history']['mean_squared_error'])
[599.5209197998047, 570.4041115897043, 510.29034205845426, 425.32058388846264]
print(history['history']['mean_squared_logarithmic_error'])
[8.624503476279122, 6.346359729766846, 3.8622902120862688, 2.1578191178185597]

在前两个 epoch 中，loss 的值等于 mean_square_error 的值，在第三和第四个 epoch 中，值变得等于 mean_square_logarithmic_error 的值，这是设置的新损失。因此，似乎使用 train_on_batch 可以更改损失函数，但是我想再次强调，这基本上是在 pytoch 上应该做的以达到相同的结果，不同之处在于 pytorch 的行为（在这种情况下和我的意见）更可靠。