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