input: def filter_out_negative(matrix):
      """Given a matrix, filter out all negative numbers by setting them to 0.
      >>> filter_out_negative([[-1, 2, -3], [-4, 5, 6], [-7, 8, 9]])
      array([[0, 2, 0],
             [0, 5, 6],
             [0, 8, 9]])
      """
output:     for i in range(len(matrix)):
          for j in range(len(matrix[i])):
              if matrix[i][j] < 0:
                  matrix[i][j] = 0
      return matrix