input: Write a python function to count the frequency of each word in a sentence.
  assert word_frequency("This is a test sentence") == {'This': 1, 'is': 1, 'a': 1, 'test': 1, 'sentence': 1}
  assert word_frequency("test test test sentence sentence") == {'test': 3, 'sentence': 2}
  assert word_frequency("Hello world Hello") == {'Hello': 2, 'world': 1}
output: def word_frequency(sentence):
      words = sentence.split()
      frequency = {}
      for word in words:
          if word in frequency:
              frequency[word] += 1
          else:
              frequency[word] = 1
      return frequency