input: Write a python function to count the number of unique elements in a list.
  assert num_unique_elements([1, 2, 3, 2, 1, 4, 7, 8, 9, 8]) == 7
  assert num_unique_elements([10, 20, 10, 30, 40, 20, 30, 50]) == 5
  assert num_unique_elements([5, 5, 5, 5, 5]) == 1
output: def num_unique_elements(lst):
      return len(set(lst))