参考网址：

http://yann.lecun.com/exdb/mnist/

http://yann.lecun.com/exdb/mnist/train-images-idx3-ubyte.gz

http://yann.lecun.com/exdb/mnist/train-labels-idx1-ubyte.gz

http://yann.lecun.com/exdb/mnist/t10k-images-idx3-ubyte.gz

http://yann.lecun.com/exdb/mnist/t10k-labels-idx1-ubyte.gz