input: Write a python function to count the number of times each word appears in a given input string.
  assert count_words("This is a test string and this is a test string again") == {'This': 1, 'is': 2, 'a': 2, 'test': 2, 'string': 2, 'and': 1, 'this': 1, 'again': 1}
  assert count_words("Hello world Hello") == {'Hello': 2, 'world': 1}
  assert count_words("Python is great and Python is fun") == {'Python': 2, 'is': 2, 'great': 1, 'and': 1, 'fun': 1}
output: def count_words(input_string):
      words = input_string.split()
      word_count = {}
      for word in words:
          if word in word_count:
              word_count[word] += 1
          else:
              word_count[word] = 1
      return word_count