input: Write a function to tokenize a sentence using nltk's word_tokenize.
  assert tokenize_sentence("This is a sample sentence") == ['This', 'is', 'a', 'sample', 'sentence']
  assert tokenize_sentence("Hello, World!") == ['Hello', ',', 'World', '!']
  assert tokenize_sentence("NLP is fun.") == ['NLP', 'is', 'fun', '.']
output: def tokenize_sentence(sentence):
      from nltk.tokenize import word_tokenize
      return word_tokenize(sentence)