>>> a = np.array([[1, 2], [3, 4]])
>>> np.mean(a) # 将上面二维矩阵的每个元素相加除以元素个数（求平均数） 
2.5
>>> np.mean(a, axis=0) # axis=0，计算所有子数组的平均值
array([ 2., 3.])
>>> np.mean(a, axis=1) # axis=1，对每一个子数组，计算它的平均值
array([ 1.5, 3.5])

X = np.mean(last_layer_delta_of_all_input, axis=0)
print 'X:', X
print 'X_shape:', X.shape
print 'own mean'
X = np.zeros((10,1))
for delta in last_layer_delta_of_all_input:
 X += delta
X /= 10
print 'own X:', X