input: Write a python function to count the number of unique elements in a list.
  assert count_unique_elements([1, 2, 3, 4, 5, 5, 4, 3, 2, 1]) == 5
  assert count_unique_elements([10, 20, 30, 40, 50, 10, 20]) == 4
  assert count_unique_elements([5, 5, 5, 5, 5]) == 1
output: def count_unique_elements(lst):
      return len(set(lst))