功能: 计算神经网络Swish激活函数

格式: netFunBySwish(x, a)

x : 为矩阵变量或者数值
a : 为数值或者1*1的矩阵变量

说明:
1. 函数执行成功返回和x同纬度的矩阵变量

2. 本函数主要计算的原理如下
Swish: 范围(-inf, inf)
$$
\mathbf{netFunBySwish}(x_i,a_i) = \dfrac{x_i}{1 + \exp(- a_ix_i)} 
$$

例子:
x =
[-0.73290256607947   -1.95092696112158    0.21744714338213    2.19789781453921
 -4.72275318588258    1.99957438581603    2.94959032789319   -4.31942455462153
 -6.10488919615973   -1.19050327068684   -6.38943656901337    2.98098757280083
 -2.76742173278119   -4.81647247463300   -3.72662812435377   -4.00467786914887 ]
y = netFunBySwish(x, 3.6)//回车得到如下结果
y =
[-0.04888790908177   -0.00173643192862    0.14923079019530    2.19709332038954
 -1.9514566078E-07    1.99808035996519    2.94951818655624   -7.6239092741E-07
 -1.7415137184E-09   -0.01616180974856   -6.5438480024E-10    2.98092245564158
 -0.00013040712674   -1.4202551253E-07   -5.5575182777E-06   -2.1948919637E-06 ]