repeat：复制元素
axis = 0 复制每行
axis = 1 复制每列
2 表示复制一遍
不设置axis，复制每个，按行展开成一行。
>>> import numpy as np
>>> a = np.mat([[1,2,3],[4,5,6]])
>>> a
matrix([[1, 2, 3],
[4, 5, 6]])
>>> a.repeat(2, axis = 0)
matrix([[1, 2, 3],
[1, 2, 3],
[4, 5, 6],
[4, 5, 6]])
>>> a.repeat(2, axis = 1)
matrix([[1, 1, 2, 2, 3, 3],
[4, 4, 5, 5, 6, 6]])
>>> a.repeat(2)
matrix([[1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6]])