input: Write a python function to count the number of each vowel in a given string.
  assert count_vowels("hello world") == {'e': 1, 'o': 2}
  assert count_vowels("beautiful day") == {'e': 1, 'a': 2, 'u': 2, 'i': 1}
  assert count_vowels("programming") == {'o': 1, 'a': 1, 'i': 1}
output: def count_vowels(s):
      vowels = 'aeiou'
      result = {v: 0 for v in vowels}
      for char in s:
          if char in vowels:
              result[char] += 1
      return {k: v for k, v in result.items() if v > 0}