如何解决如何在 Tensorflow Keras API 中使用配对数据集样本创建联合损失？
我正在尝试训练一个自动编码器，其约束强制一个或多个隐藏/编码节点/神经元具有可解释的值。我的训练方法使用成对图像（尽管在训练后模型应该对单个图像进行操作）并利用联合损失函数，其中包括（1）每个图像的重建损失和（2）隐藏/编码向量，来自两个图像中的每一个。
我创建了一个类似的简单玩具问题和模型来使这一点更清晰。在玩具问题中，给自编码器一个长度为 3 的向量作为输入。编码使用一个密集层来计算均值（标量）和另一个密集层来计算向量的其他一些表示（根据我的构造，它可能只会学习一个单位矩阵，即复制输入向量）。见下图。隐藏层的最低节点用于计算输入向量的均值。除了必须适应与输入匹配的重建之外，其余隐藏节点不受约束。
下图展示了我希望如何使用配对图像训练模型。 “MSE”是均方误差，尽管实际函数的身份对于我在这里提出的问题并不重要。损失函数是重建损失和均值估计损失之和。
我尝试创建 (1) 一个 tf.data.Dataset 来生成配对向量，(2) 一个 Keras 模型，以及 (3) 一个自定义损失函数。但是，我无法理解如何在这种特殊情况下正确执行此操作。
我无法让 Model.fit() 正确运行，也无法按预期将模型输出与数据集目标相关联。请参阅下面的代码和错误。任何人都可以帮忙吗？我在 Google 和 stackoverflow 上进行了很多搜索，但仍然不明白如何实现这一点。
import tensorflow as tf
import os
os.environ['TF_CPP_MIN_LOG_LEVEL'] = '3'
DTYPE = tf.dtypes.float32
N_VEC = 3
def my_generator(n):
while True:
# Create two identical vectors of length,except with different means.
# An internal layer (single neuron) of the model should predict the
# mean of the input vector. To train it to do so,with paired
# vector inputs,use a loss function that penalizes incorrect
# predictions of the difference of the means of two input vectors.
input_vec1 = tf.random.normal((n,),dtype=DTYPE)
target_mean_diff = tf.random.normal((1,dtype=DTYPE)
input_vec2 = input_vec1 + target_mean_diff
# Model is a constrained autoencoder. Output targets are
# identical to the input vectors. Including them as explicit
# targets in this generator,for generalization.
target_vec1 = tf.identity(input_vec1)
target_vec2 = tf.identity(input_vec2)
yield ({'input_vec1':input_vec1,'input_vec2':input_vec2},{'target_vec1':target_vec1,'target_vec2':target_vec2,'target_mean_diff':target_mean_diff})
def my_dataset(n,batch_size=4):
ds = tf.data.Dataset.from_generator(my_generator,output_signature=({'input_vec1':tf.TensorSpec(shape=(n,dtype=DTYPE),'input_vec2':tf.TensorSpec(shape=(n,dtype=DTYPE)},{'target_vec1':tf.TensorSpec(shape=(n,'target_vec2':tf.TensorSpec(shape=(n,'target_mean_diff':tf.TensorSpec(shape=(1,dtype=DTYPE)}),args=(n,))
ds = ds.batch(batch_size)
return ds
## Do a brief test using the Dataset
ds = my_dataset(N_VEC,batch_size=4)
ds_iter = iter(ds)
dict_inputs,dict_targets = next(ds_iter)
print(dict_inputs)
print(dict_targets)
## Define the Model
layer_encode_vec = tf.keras.layers.Dense(N_VEC,activation=None,name='encode_vec')
layer_decode_vec = tf.keras.layers.Dense(N_VEC,name='decode_vec')
layer_encode_mean = tf.keras.layers.Dense(1,name='encode_mean')
layer_decode_mean = tf.keras.layers.Dense(N_VEC,name='decode_mean')
input1 = tf.keras.Input(shape=(N_VEC,name='input_vec1')
input2 = tf.keras.Input(shape=(N_VEC,name='input_vec2')
vec_encoded1 = layer_encode_vec(input1)
vec_encoded2 = layer_encode_vec(input2)
mean_encoded1 = layer_encode_mean(input1)
mean_encoded2 = layer_encode_mean(input2)
mean_diff = mean_encoded2 - mean_encoded1
pred_vec1 = layer_decode_vec(vec_encoded1) + layer_decode_mean(mean_encoded1)
pred_vec2 = layer_decode_vec(vec_encoded2) + layer_decode_mean(mean_encoded2)
model = tf.keras.Model(inputs=[input1,input2],outputs=[pred_vec1,pred_vec2,mean_diff])
print(model.summary())
## Define the joint loss function
def loss_total(y_true,y_pred):
loss_reconstruct = tf.reduce_mean(tf.keras.MSE(y_true[0],y_pred[0]))/2 + \
tf.reduce_mean(tf.keras.MSE(y_true[1],y_pred[1]))/2
loss_mean = tf.reduce_mean(tf.keras.MSE(y_true[2],y_pred[2]))
return loss_reconstruct + loss_mean
## Compile model
optimizer = tf.keras.optimizers.Adam(lr=0.01)
model.compile(optimizer=optimizer,loss=loss_total)
## Train model
history = model.fit(x=ds,epochs=10,steps_per_epoch=10)
输出：来自数据集的示例批次：
{'input_vec1': <tf.Tensor: shape=(4,3),dtype=float32,numpy=
array([[-0.53022575,-0.02389329,0.32843253],[-0.61793506,-0.8276422,-1.3469328 ],[-0.5401968,0.3141346,-1.3638284 ],[-1.2189807,0.23848908,0.75108534]],dtype=float32)>,'input_vec2': <tf.Tensor: shape=(4,numpy=
array([[-0.23415083,0.27218163,0.6245074 ],[-0.57636774,-0.7860749,-1.3053654 ],[ 0.65463066,1.508962,-0.16900098],[-0.49326736,0.9642024,1.4767987 ]],dtype=float32)>}
{'target_vec1': <tf.Tensor: shape=(4,'target_vec2': <tf.Tensor: shape=(4,'target_mean_diff': <tf.Tensor: shape=(4,1),numpy=
array([[0.29607493],[0.04156734],[1.1948274 ],[0.7257133 ]],dtype=float32)>}
输出：模型摘要：
Model: "model"
__________________________________________________________________________________________________
Layer (type) Output Shape Param # Connected to
==================================================================================================
input_vec1 (InputLayer) [(None,3)] 0
__________________________________________________________________________________________________
input_vec2 (InputLayer) [(None,3)] 0
__________________________________________________________________________________________________
encode_vec (Dense) (None,3) 12 input_vec1[0][0]
input_vec2[0][0]
__________________________________________________________________________________________________
encode_mean (Dense) (None,1) 4 input_vec1[0][0]
input_vec2[0][0]
__________________________________________________________________________________________________
decode_vec (Dense) (None,3) 12 encode_vec[0][0]
encode_vec[1][0]
__________________________________________________________________________________________________
decode_mean (Dense) (None,3) 6 encode_mean[0][0]
encode_mean[1][0]
__________________________________________________________________________________________________
tf.__operators__.add (TFOpLambd (None,3) 0 decode_vec[0][0]
decode_mean[0][0]
__________________________________________________________________________________________________
tf.__operators__.add_1 (TFOpLam (None,3) 0 decode_vec[1][0]
decode_mean[1][0]
__________________________________________________________________________________________________
tf.math.subtract (TFOpLambda) (None,1) 0 encode_mean[1][0]
encode_mean[0][0]
==================================================================================================
Total params: 34
Trainable params: 34
Non-trainable params: 0
__________________________________________________________________________________________________
输出：调用model.fit()时的错误信息：
Epoch 1/10
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
...
ValueError: Found unexpected keys that do not correspond to any
Model output: dict_keys(['target_vec1','target_vec2','target_mean_diff']).
Expected: ['tf.__operators__.add','tf.__operators__.add_1','tf.math.subtract']
解决方法
您可以像这样将 dict 和 Model 的 inputs 传递给 outputs：
model = tf.keras.Model(
inputs={"input_vec1": input1,"input_vec2": input2},outputs={
"target_vec1": pred_vec1,"target_vec2": pred_vec2,"target_mean_diff": mean_diff,},)
这避免了必须命名输出层。
对于损失，它目前将 loss_total 分别应用于 3 个输出中的每一个并求和以获得最终损失，这不是您想要的。因此，您可以单独列出每个损失：
model.compile(
optimizer=optimizer,loss={"target_vec1": "mse","target_vec2": "mse","target_mean_diff": "mse"},loss_weights={"target_vec1": 0.5,"target_vec2": 0.5,"target_mean_diff": 1},)
或者您可以使用采用 dict 输入的修改后的损失函数手动训练模型。类似的东西：
def loss_total(y_true,y_pred):
loss_reconstruct = (
tf.reduce_mean(tf.keras.losses.MSE(y_true["target_vec1"],y_pred["target_vec1"])) / 2
+ tf.reduce_mean(tf.keras.losses.MSE(y_true["target_vec2"],y_pred["target_vec2"])) / 2
)
loss_mean = tf.reduce_mean(tf.keras.losses.MSE(y_true["target_mean_diff"],y_pred["target_mean_diff"]))
return loss_reconstruct + loss_mean
for epoch in range(10):
for batch,(x,y) in zip(range(10),ds):
with tf.GradientTape() as tape:
outputs = model(x,training=True)
loss = loss_total(y,outputs)
trainable_vars = model.trainable_variables
gradients = tape.gradient(loss,trainable_vars)
optimizer.apply_gradients(zip(gradients,trainable_vars))
print(f"Batch: {batch},loss: {loss.numpy()}")
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