input: Write a python function to count unique words in a sentence.
  assert count_unique_words("Hello world. This is a sample sentence.") == 7
  assert count_unique_words("This is a test. This test is only a test.") == 6
  assert count_unique_words("One fish two fish red fish blue fish.") == 5
output: def count_unique_words(sentence):
      words = sentence.lower().split()
      unique_words = set(word.strip('.,!?:;') for word in words)
      return len(unique_words)