input: Write a python function to count the number of vowels in each word from a list of words.
  assert num_vowels(["hello", "world"]) == {"hello": 2, "world": 1}
  assert num_vowels(["apple", "banana", "cherry"]) == {"apple": 2, "banana": 3, "cherry": 2}
  assert num_vowels(["sky", "fly", "by"]) == {"sky": 0, "fly": 0, "by": 0}
output: def num_vowels(words):
      vowels = "aeiou"
      vowel_count = {}
      for word in words:
          count = sum(1 for char in word if char in vowels)
          vowel_count[word] = count
      return vowel_count