input: Write a python function to assign a label to a given text based on predefined assignment words using nltk.
  assert assign_label("The cat is on the sofa.")=="Animal"
  assert assign_label("He sat on the sofa.")=="Furniture"
  assert assign_label("A cat is here.")=="Animal"
  assert assign_label("No relevant words.")==None
output: def assign_label(text):
      import nltk
      from nltk.tokenize import word_tokenize
      
      nltk.download('punkt')
      
      assignment_words = {
          "Animal": ["cat", "dog", "horse", "bird"],
          "Furniture": ["sofa", "table", "chair", "desk"]
      }
      
      tokens = word_tokenize(text.lower())
      
      for label, words in assignment_words.items():
          if any(word in tokens for word in words):
              return label
      
      return None