input: Write a python function to tokenize user input using nltk library.
  assert tokenize("Hello! How are you?") == ["Hello", "!", "How", "are", "you", "?"]
  assert tokenize("What is your name?") == ["What", "is", "your", "name", "?"]
  assert tokenize("Python is amazing!") == ["Python", "is", "amazing", "!"]
output: def tokenize(text):
      import nltk
      nltk.download('punkt')
      from nltk.tokenize import word_tokenize
      return word_tokenize(text)