cumsum函数

>>> a = np.array([[1,2,3], [4,5,6]])

>>> a

array([[1, 2, 3],

[4, 5, 6]])

>>> np.cumsum(a)

array([ 1,  3,  6, 10, 15, 21]) # 计算累加和，1=1,3=1+2,6=1+2+3....