input: Write a python function to find unique words in a string.
  assert find_unique_words("hello world hello") == {"hello", "world"}
  assert find_unique_words("one two three one two") == {"one", "two", "three"}
  assert find_unique_words("a b c d a b") == {"a", "b", "c", "d"}
output: def find_unique_words(s):
      return set(s.split())