input: Write a python function to reverse each word in a sentence.
  assert reverse_words("This is a string") == "sihT si a gnirts"
  assert reverse_words("Hello World") == "olleH dlroW"
  assert reverse_words("Python is fun") == "nohtyP si nuf"
output: def reverse_words(sentence):
      return ' '.join(word[::-1] for word in sentence.split())