Add a numerical stable version of softmax

[praisethemoon]

Add a numerical stable version of softmax

https://deepnotes.io/softmax-crossentropy
http://www.nowozin.net/sebastian/blog/streaming-log-sum-exp-computation.html

[praisethemoon]

Another solution for sigmoid: https://timvieira.github.io/blog/post/2014/02/11/exp-normalize-trick/