说明：  用MATLAB搭建一个可自定义层数的深度神经网络，并以MNIST为例进行网络训练、验证、误差可视化展示。不仅限制在该手写数字库，所搭建的DNN也可以用于其他数据。
(A depth neural network with user-defined layers is built with MATLAB, and MNIST is taken as an example for network training, verification and error visualization. It is not only limited to the handwritten numeral library, but also can be used for other data.)