input: Write a python function to compute the 2,1-norm of a given matrix.
  assert norm_2_1(np.array([[1,1],[2,2]])) == 4.242640687119285
  assert norm_2_1(np.array([[3,4],[1,2]])) == 7.0710678118654755
  assert norm_2_1(np.array([[0,0],[0,0]])) == 0
output: import numpy as np
  
  def norm_2_1(matrix):
      return np.sum(np.linalg.norm(matrix, ord=2, axis=0))