logistic回归为什么要使用sigmoid函数
https://www.baidu.com/link?url=LnDjrhLG7Fx6YVgR9WljUILkPZrIzOR402wr2goIS-ARtDv9TwZ2VYVbY74fyVpQlE22nZsMSJLhvNXhxnPNWQpKtPUlBk4SGM8EGjBX1WK&wd=&eqid=b9f03dd400028a0f000000025bc45630
https://blog.csdn.net/jinesse/article/details/50553870