input: Write a function to remove duplicate elements from a list and return the unique elements as a set.
  assert remove_duplicates([1, 2, 3, 2, 1, 4]) == {1, 2, 3, 4}
  assert remove_duplicates([5, 5, 5, 5, 5]) == {5}
  assert remove_duplicates([]) == set()
  assert remove_duplicates([10, 20, 20, 30, 40, 40, 50]) == {10, 20, 30, 40, 50}
output: def remove_duplicates(lst):
      return set(lst)