如何理解范数和正则项

参考：https://zhuanlan.zhihu.com/p/26884695
https://medium.com/mlreview/l1-norm-regularization-and-sparsity-explained-for-dummies-5b0e4be3938a
https://rorasa.wordpress.com/2012/05/13/l0-norm-l1-norm-l2-norm-l-infinity-norm/