input: Write a python function to count the number of occurrences of each word in a given string.
  assert count_words("apple banana apple orange banana apple") == {'apple': 3, 'banana': 2, 'orange': 1}
  assert count_words("hello world hello") == {'hello': 2, 'world': 1}
  assert count_words("test test test test test") == {'test': 5}
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