Sigmoid函数的替代：overflow encountered in exp in computing the logistic function

RuntimeWarning: overflow encountered in exp in computing the logistic function

如下是sigmoid函数的标准写法，可是若是x很大或致使函数exp(-x)溢出

def logistic_function(x):
#     x = np.float64(x)
    return 1.0 / (1.0 + np.exp(-x))

安全的替代写法以下：

def logistic_function(x):
    return .5 * (1 + np.tanh(.5 * x))