code to show F-Principle in the DNN training

Reference:

1 Training behavior of deep neural network in frequency domain: https://arxiv.org/abs/1807.01251

2 Frequency Principle: Fourier Analysis Sheds Light on Deep Neural Networks: https://arxiv.org/abs/1901.06523

3 Explicitizing an Implicit Bias of the Frequency Principle in Two-layer Neural Networks: https://arxiv.org/abs/1905.10264

4 Theory of the Frequency Principle for General Deep Neural Networks: https://arxiv.org/abs/1906.09235