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Title: Decoupling the Layers in Residual Networks

Paper content:
We propose a Warped Residual Network (WarpNet) using a parallelizable warp operator for forward and backward propagation to distant layers that trains faster than the original residual neural network.
We apply a perturbation theory on residual networks and decouple the interactions between residual units.
The resulting warp operator is a first order approximation of the output over multiple layers.
The first order perturbation theory exhibits properties such as binomial path lengths and exponential gradient scaling found experimentally by Veit et al (2016).
We demonstrate through an extensive performance study that the proposed network achieves comparable predictive performance to the original residual network with the same number of parameters, while achieving a significant speed-up on the total training time.
As WarpNet performs model parallelism in residual network training in which weights are distributed over different GPUs, it offers speed-up and capability to train larger networks compared to original residual networks.
Deep Convolution Neural Networks (CNN) have been used in image recognition tasks with great success.
Since AlexNet BID6 , many other neural architectures have been proposed to achieve start-of-the-art results at the time.
Some of the notable architectures include, VGG BID7 , Inception and Residual networks (ResNet) BID3 .Training
a deep neural network is not an easy task. As the gradient
at each layer is dependent upon those in higher layers multiplicatively, the gradients in earlier layers can vanish or explode, ceasing the training process. The gradient vanishing
problem is significant for neuron activation functions such as the sigmoid, where the gradient approaches zero exponentially away from the origin on both sides. The standard approach
to combat vanishing gradient is to apply Batch Normalization (BN) BID5 followed by the Rectified Linear Unit (ReLU) BID1 activation. More recently, skip connections
BID9 have been proposed to allow previous layers propagate relatively unchanged. Using this methodology the authors
in BID9 were able to train extremely deep networks (hundreds of layers) and about one thousand layers were trained in residual networks BID3 .As the number of layers grows large
, so does the training time. To evaluate the neural network's output
, one needs to propagate the input of the network layer by layer in a procedure known as forward propagation. Likewise, during training, one needs to
propagate the gradient of the loss function from the end of the network to update the model parameters, or weights, in each layer of the network using gradient descent. The complexity of forward and propagation
is O(K), where K is the number of layers in the network. To speed up the process, one may ask if there
exist a shallower network that accurately approximates a deep network so that training time is reduced. In this work we show that there indeed exists
a neural network architecture that permits such an approximation, the ResNet.Residual networks typically consist of a long chain of residual units. Recent investigations suggest that ResNets behave
as an ensemble of shallow networks BID11 . Empirical evidence supporting this claim includes
one that shows randomly deactivating residual units during training (similar to drop-out BID8 ) appears to improve performance BID4 . The results imply that the output of a residual unit
is just a small perturbation of the input. In this work, we make an approximation of the ResNet
by using a series expansion in the small perturbation. We find that merely the first term in the series expansion
is sufficient to explain the binomial distribution of path lengths and exponential gradient scaling experimentally observed by BID11 . The approximation allows us to effectively estimate the output
of subsequent layers using just the input of the first layer and obtain a modified forward propagation rule. We call the corresponding operator the warp operator. The backpropagation
rule is obtained by differentiating the warp operator
. We implemented a network using the warp operator and found that our network
trains faster on image classification tasks with predictive accuracies comparable to those of the original ResNet.

assistant: text: We propose the Warped Residual Network using a parallelizable warp operator for forward and backward propagation to distant layers that trains faster than the original residual neural network.