input: Write a python function to count the frequency of unique words in a given string.
  assert unique_words_freq("Hello world, hello world") == {'Hello': 1, 'world,': 1, 'hello': 1, 'world': 1}
  assert unique_words_freq("one fish two fish red fish blue fish") == {'one': 1, 'fish': 4, 'two': 1, 'red': 1, 'blue': 1}
  assert unique_words_freq("test test test test") == {'test': 4}
output: def unique_words_freq(s):
      words = s.split()
      freq = {}
      for word in words:
          if word in freq:
              freq[word] += 1
          else:
              freq[word] = 1
      return freq