input: Write a python function to reverse each word in a given string.
  assert reverse_words("hello world") == "olleh dlrow"
  assert reverse_words("python is fun") == "nohtyp si nuf"
  assert reverse_words("openai gpt") == "ianepo tpg"
output: def reverse_words(s):
      return ' '.join(word[::-1] for word in s.split())