input: Write a python function to count the frequency of each word in a sentence.
  assert count_words("This is a test. This test is only a test.") == {'This': 2, 'is': 2, 'a': 2, 'test.': 2, 'test': 1, 'only': 1}
  assert count_words("Hello world, hello!") == {'Hello': 1, 'world,': 1, 'hello!': 1}
  assert count_words("One fish two fish red fish blue fish") == {'One': 1, 'fish': 4, 'two': 1, 'red': 1, 'blue': 1}
output: def count_words(sentence):
      words = sentence.split()
      word_count = {}
      for word in words:
          word_count[word] = word_count.get(word, 0) + 1
      return word_count