input: Write a python function to count the number of words in a given sentence.
  assert Sentence("This is a test sentence.").get_num_words() == 5
  assert Sentence("Another test.").get_num_words() == 2
  assert Sentence("").get_num_words() == 0
output: class Sentence:
      def __init__(self, text):
          self.text = text
      
      def get_num_words(self):
          return len(self.text.split())