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