input: Write a python function to count the number of unique words in a given string.
  assert word_count("The quick brown fox jumped over the lazy dog")==8
  assert word_count("hello world hello")==2
  assert word_count("Python programming is fun")==4
output: def word_count(s):
      words = s.split()
      unique_words = set(words)
      return len(unique_words)