max函数

无论是几维，都只返回一个最大的值

>>> a = [1,2,3]
>>> np.max(a)
3
>>> a = [[2,1],[3,4]]
>>> np.max(a)
4
>>> a = [[2,1],[3,4],[2,6]]
>>> np.max(a)
6
>>> a = [[2,1,6],[3,4,3],[2,6,0]]
>>> np.max(a)
6