https://medium.com/@azimkhan8018/a-beginners-guide-to-deep-learning-with-mnist-dataset-0894f7183344
https://ranasinghiitkgp.medium.com/principal-component-analysis-pca-with-code-on-mnist-dataset-da7de0d07c22
https://github.com/mkosaka1/MNIST_PCA_CNN/blob/main/PCA%20%2B%20CNN.ipynb
https://gist.github.com/tommct/1490cdf856d745ba41c1ac99ada2b579