Add NN function

[Elvis-ever]

Go through the practice results, it looks that NN need an activation function (like relu), otherwise it's just a trivial linear model.By applying relu, the accuracy is neural_network.py: 92% -> 95%,
Accuracy for neural_network_raw.py: 92% -> 94%.And the learning rate is too large, even for example. In practice, it is usually set between 1e-2 and 1e-4.

[xy-always]

已经收到您的邮件。

[jiandandema]

谢谢您给马坤发送邮件，我会及时查看的，谢谢，辛苦了