如何对numpy 矩阵进行通道间求均值

import numpy as np
a = np.array([
              [[1, 2],
              [3, 4]],
              [[5,6],
               [7,8]],
                [[1, 2],
              [3, 4]]
              ])
print(a.shape) # (3, 2, 2)
print(np.mean(a, axis=0))
"""
[[2.33333333 3.33333333]
 [4.33333333 5.33333333]]
"""
print(np.mean(a, axis=1))
"""
[[2. 3.]
 [6. 7.]
 [2. 3.]]
"""
print(np.mean(a, axis=2))
"""
[[1.5 3.5]
 [5.5 7.5]
 [1.5 3.5]]
"""