Loads the MNIST dataset.
path: path where to cache the dataset locally (relative to ~/.keras/datasets).
Tuple of Numpy arrays: `(x_train, y_train), (x_test, y_test)`.
License: Yann LeCun and Corinna Cortes hold the copyright of MNIST dataset, which is a derivative work from original NIST datasets. MNIST dataset is made available under the terms of the Creative Commons Attribution-Share Alike 3.0 license.