input: Write a function to find the smallest element in a 2D list.
  assert find_smallest_element_2D([[1, 2, 3], [4, 5, 6], [7, 8, 9]]) == 1
  assert find_smallest_element_2D([[10, 20, 30], [5, 15, 25], [35, 45, 55]]) == 5
  assert find_smallest_element_2D([[11, 22, 33], [44, 55, 66], [7, 88, 99]]) == 7
output: def find_smallest_element_2D(lst):
      return min(min(row) for row in lst)