input: Write a python function to tokenize a sentence using NLTK.
  assert tokenize_sentence("The cat sat on the mat.") == ['The', 'cat', 'sat', 'on', 'the', 'mat', '.']
  assert tokenize_sentence("Hello, world!") == ['Hello', ',', 'world', '!']
  assert tokenize_sentence("NLP is fun.") == ['NLP', 'is', 'fun', '.']
output: def tokenize_sentence(sentence):
      import nltk
      from nltk.tokenize import word_tokenize
      nltk.download('punkt', quiet=True)
      return word_tokenize(sentence)