ReLU関数
https://qiita-user-contents.imgix.net/https%3A%2F%2Fqiita-image-store.s3.ap-northeast-1.amazonaws.com%2F0%2F192073%2F37688657-d49e-f643-faaf-a3b161c49945.png?ixlib=rb-1.2.2&auto=compress%2Cformat&gif-q=60&s=2f96d98444b23eca5317d88fa69e1789
f(x) = x (x >= 0) : 0 (x < 0)
#テーマ5