input: Write a python function to convert a word to Pig Latin.
  assert pig_latin("apple") == "appleay"
  assert pig_latin("banana") == "ananabay"
  assert pig_latin("orange") == "orangeay"
  assert pig_latin("grape") == "rapegray"
output: def pig_latin(word):
      vowels = 'aeiou'
      if word[0] in vowels:
          return word + 'ay'
      else:
          return word[1:] + word[0] + 'ay'