e.g.
1) auto ds = load_images("catsdogs"); // ds.first: training set, ds.second: validation set
2) auto ds = load_images("cifar10"); // ds.first: training set, ds.second: validation set
3) auto ds = load_images("mnist"); // ds.first: training set, ds.second: validation set
4) auto ds = load_images("flowers"); // ds.first: training set, ds.second: validation set