来源：https://blog.csdn.net/weixin_41793877/article/details/84700875
[https://blog.csdn.net/m0_37772174/article/details/81356434?depth_1-utm_source=distribute.pc_relevant_right.none-task&utm_source=distribute.pc_relevant_right.none-task]
http://www.52nlp.cn/lda-math-mcmc-%E5%92%8C-gibbs-sampling1
关于多维正态变量的采样
