损失函数
1 Square loss
2 Hinge loss
3 Logistic loss
4 Cross entropy loss
Using the alternative label convention,so that,
the cross entropy loss is defined as
x = np.linspace(-3, 3, 1000)
y_logit = np.log(1 + np.exp(-x)) / math.log(2)
y_01 = x < 0
y_hinge = 1 - x
y_hinge[y_hinge < 0] = 0