input: Write a python function to reverse each word in a string.
  assert word_reverse("hello world") == "olleh dlrow "
  assert word_reverse("python programming") == "nohtyp gnimmargorp "
  assert word_reverse("reverse words") == "esrever sdrow "
output: def word_reverse(s):
      return ' '.join(word[::-1] for word in s.split()) + ' '