重新审视神经正切核在宽深网络中的作用,发现其仍可描述非平凡输出。
Revisiting the Neural Tangent Kernel: the role of large width and depth
- 发现单个隐藏层激活虽不变,但整体偏差的范数不为零
- 理论上证明深层网络最后一层激活不恒定,打破懒惰训练假设
- 通过缩放深度与停止时间,实现大数据集上的稳定非平凡输出
过参数化的全连接神经网络在梯度下降训练下,当宽度、学习率和参数初始化满足标准缩放条件时,行为类似于核模型。在无限宽且学习率趋近于零的极限下,神经正切核(NTK)提供了一个依赖于架构和激活函数的闭式解,且在整个训练过程中保持不变,称为“懒惰训练”或“懒惰区间”。先前研究认为,无限宽网络中隐藏神经元激活几乎不变,且随着深度增加,NTK诱导的解变为与数据无关的平凡解。本文挑战这两个结论,实验表明:虽然个体隐藏层神经元的激活偏差消失,但这些偏差的总体范数却不为零;理论证明最后一层激活并不恒定。此外,通过适当缩放深度和停止时间,在无限宽的ReLU网络中,可在大数据集上获得良好表现且非平凡的输出。我们在大规模数据集上验证了该行为的稳定性,并揭示了使结果推广到其他核的关键性质。
原文摘要 · Abstract (English)
Overparameterized fully-connected neural networks have been shown to behave like kernel models when trained with gradient descent, assuming standard scaling conditions on the width, the learning rate, and the parameter initialization. In the limit of infinitely large widths and infinitesimal learning rate, the obtained kernel provides a description of the learned model's output via a closed-form solution dependent on the architecture and the activation function. The Neural Tangent Kernel, central to this description, remains constant throughout training, a phenomenon that is referred to as ``lazy training'' or within the ``lazy regime''. Prior works show that the ``lazy regime'' leads to non-varying hidden neuron activations in infinitely-wide networks. Moreover, as infinitely-wide networks increase in depth, the Neural Tangent Kernel induces a closed-form solution that is data-independent, hence trivial. The Neural Tangent Kernel seemingly fails to describe the complexity of overparameterized neural networks on two distinct axes: large widths and large depths. In this work, we challenge these two conclusions and open the door to re-evaluating the Neural Tangent Kernel's role in describing the output of overparameterized neural networks. Specifically, we show experimentally that while deviations in the activations of individual hidden neurons vanish, the aggregate norm of these deviations does not. We support this finding with a theoretical result showing that the activations of the last hidden layer do not remain constant. Furthermore, we demonstrate that properly scaling the depth and stopping time in infinitely-wide ReLU networks yields a well-behaved, non-trivial output at large dataset sizes. We empirically evaluate the stability of this behavior on large datasets, and we describe the essential properties that enable the generalization of our results to other kernels.
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