input: Write a function to encode the elements of a 2D list into strings using a similar approach as DataFrameEncoder class.
  assert encode_2d_list([[1, 2], [3, 4]]) == [['1', '2'], ['3', '4']]
  assert encode_2d_list([[5, 6], [7, 8]]) == [['5', '6'], ['7', '8']]
  assert encode_2d_list([[9, 10], [11, 12]]) == [['9', '10'], ['11', '12']]
output: def encode_2d_list(lst):
      return [[str(element) for element in row] for row in lst]