symmetric version: called Hopfield Net
if y <-- minus y, energy won‘t change!
the second last line‘s first "-" is not the minus sign
https://zhuanlan.zhihu.com/p/21539285
CMU Deep Learning 2018 by Bhiksha Raj 学习记录(20) Lecture 20: Hopfield Networks 1
原文：https://www.cnblogs.com/ecoflex/p/8971351.html