input: Write a python function to tokenize a given sentence using NLTK's word_tokenize method.
  assert tokenize_sentence('This is a test sentence.') == ['This', 'is', 'a', 'test', 'sentence', '.']
  assert tokenize_sentence('Hello world!') == ['Hello', 'world', '!']
  assert tokenize_sentence('NLTK is great for text processing.') == ['NLTK', 'is', 'great', 'for', 'text', 'processing', '.']
output: def tokenize_sentence(sentence):
      from nltk.tokenize import word_tokenize
      return word_tokenize(sentence)