input: Write a function to get all unique largest elements from a list and sort them in descending order.
  assert largest_elements([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])==[10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
  assert largest_elements([10, 20, 45, 67, 84, 93, 93])==[93, 84, 67, 45, 20, 10]
  assert largest_elements([5, 7, 9, 8, 6, 4, 3, 7])==[9, 8, 7, 6, 5, 4, 3]
output: def largest_elements(lst):
      unique_elements = list(set(lst))
      unique_elements.sort(reverse=True)
      return unique_elements