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 is amazing") == "IAnepO si gnizama"
output: def reverse_words(s):
      return ' '.join(word[::-1] for word in s.split())