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