http://fancyerii.github.io/2019/06/30/xlnet-theory/
http://fancyerii.github.io/2019/06/30/xlnet-codes/
http://fancyerii.github.io/2019/07/14/xlnet-codes2/
http://fancyerii.github.io/2019/07/20/xlnet-codes3/
http://fancyerii.github.io/2019/08/16/xlnet-codes4/