input: Write a python function to count the occurrences of each word in a string.
  assert count_words("hello world and hello universe") == {'hello': 2, 'world': 1, 'and': 1, 'universe': 1}
  assert count_words("one fish two fish red fish blue fish") == {'one': 1, 'fish': 4, 'two': 1, 'red': 1, 'blue': 1}
  assert count_words("repeat repeat repeat") == {'repeat': 3}
output: def count_words(s):
      words = s.split()
      word_count = {}
      for word in words:
          if word in word_count:
              word_count[word] += 1
          else:
              word_count[word] = 1
      return word_count