input: Write a python function to count the number of unique words in a given string.
  assert count_unique_words("hello world") == 2
  assert count_unique_words("hello hello world") == 2
  assert count_unique_words("apple orange banana apple grape") == 4
output: def count_unique_words(s):
      words = s.split()
      unique_words = set(words)
      return len(unique_words)