input: Write a python function to tokenize sentences using nltk.
  assert tokenize_sentences("Hello, world!") == ['Hello', ',', 'world', '!']
  assert tokenize_sentences("This is a test sentence.") == ['This', 'is', 'a', 'test', 'sentence', '.']
  assert tokenize_sentences("I am learning Python.") == ['I', 'am', 'learning', 'Python', '.']
output: def tokenize_sentences(text):
      import nltk
      from nltk.tokenize import word_tokenize
      nltk.download('punkt')
      return word_tokenize(text)