input: Write a python function to split a sentence into unique words and return them as a space-separated string.
  assert split_unique_words("this is a test sentence this is only a test") == "this is a test sentence only"
  assert split_unique_words("hello world world hello") == "world hello"
  assert split_unique_words("unique words in this sentence are unique") == "unique words in this sentence are"
output: def split_unique_words(sentence):
      words = sentence.split()
      seen = set()
      unique_words = []
      for word in words:
          if word not in seen:
              seen.add(word)
              unique_words.append(word)
      return ' '.join(unique_words)