input: Write a Python function to sum all the elements of a 2D list.
  assert sum_elements_2dlist([[1, 2, 3], [4, 5, 6], [7, 8, 9]]) == 45
  assert sum_elements_2dlist([[0, 0, 0], [0, 0, 0], [0, 0, 0]]) == 0
  assert sum_elements_2dlist([[1, -1], [-1, 1]]) == 0
output: def sum_elements_2dlist(lst):
      return sum(sum(inner_list) for inner_list in lst)