input: Write a python function to reverse each word in a given sentence.
  assert reverse_words('This is a test') == 'sihT si a tset'
  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())