input: Write a python function to extract unique words from a sentence.
  assert extract_unique_words("the quick brown fox jumps over the lazy dog") == {'the', 'quick', 'brown', 'fox', 'jumps', 'over', 'lazy', 'dog'}
  assert extract_unique_words("hello world hello") == {'hello', 'world'}
  assert extract_unique_words("test test test test") == {'test'}
output: def extract_unique_words(sentence):
      return set(sentence.split())