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