input: Write a python function to perform sentiment analysis on a given sentence using NLTK's VADER sentiment analyzer.
  assert sentiment_analysis("I love the way you smile.")['compound'] > 0
  assert sentiment_analysis("I hate the way you frown.")['compound'] < 0
  assert sentiment_analysis("This is a very neutral statement.")['compound'] == 0
output: def sentiment_analysis(sentence):
      from nltk.sentiment.vader import SentimentIntensityAnalyzer
      import nltk
      nltk.download('vader_lexicon')
      
      sid = SentimentIntensityAnalyzer()
      return sid.polarity_scores(sentence)