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("a b c d") == ['a', 'b', 'c', 'd']
output: def reverse_words(s):
      return [word[::-1] for word in s.split()]