input: Write a python function to count the frequency of each word in a given sentence.
  assert word_count("hello world hello") == {"hello": 2, "world": 1}
  assert word_count("test the test") == {"test": 2, "the": 1}
  assert word_count("a a a b b c") == {"a": 3, "b": 2, "c": 1}
output: def word_count(sentence):
      words = sentence.split()
      frequency = {}
      for word in words:
          frequency[word] = frequency.get(word, 0) + 1
      return frequency