input: Write a function to count the occurrences of each word in a string.
  assert count_word_occurrences("hello world hello") == {'hello': 2, 'world': 1}
  assert count_word_occurrences("test the test case") == {'test': 2, 'the': 1, 'case': 1}
  assert count_word_occurrences("one fish two fish red fish blue fish") == {'one': 1, 'fish': 4, 'two': 1, 'red': 1, 'blue': 1}
output: def count_word_occurrences(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