input: Write a python function to count the frequency of words in a given sentence using NLTK.
  assert word_frequency("The quick brown fox jumps over the lazy dog") == {'The': 1, 'quick': 1, 'brown': 1, 'fox': 1, 'jumps': 1, 'over': 1, 'the': 1, 'lazy': 1, 'dog': 1}
  assert word_frequency("Hello world! Hello universe!") == {'Hello': 2, 'world': 1, '!': 2, 'universe': 1}
  assert word_frequency("This is a test. This test is only a test.") == {'This': 2, 'is': 2, 'a': 2, 'test': 3, '.': 2, 'only': 1}
output: def word_frequency(sentence):
      import nltk
      from collections import Counter
      from nltk.tokenize import word_tokenize
      
      nltk.download('punkt', quiet=True)
      
      words = word_tokenize(sentence)
      frequency = Counter(words)
      
      return dict(frequency)