input: 
  def visualize_twitter_data(twitter_data):
      """
      Visually summarizes Twitter data by plotting the number of tweets per author, 
      the number of likes per tweet, and the number of retweets per tweet.
  
      >>> visualize_twitter_data([{'author': 'User1', 'likes': 10, 'retweets': 5}, {'author': 'User2', 'likes': 20, 'retweets': 10}])
      # Displays three bar plots: one for tweet authors, one for number of likes, and one for number of retweets
      """
  
output:     import matplotlib.pyplot as plt
      from collections import defaultdict
  
      authors = defaultdict(int)
      likes = []
      retweets = []
  
      for tweet in twitter_data:
          authors[tweet['author']] += 1
          likes.append(tweet['likes'])
          retweets.append(tweet['retweets'])
  
      fig, axs = plt.subplots(3, 1, figsize=(10, 15))
  
      axs[0].bar(authors.keys(), authors.values())
      axs[0].set_title('Number of Tweets per Author')
      axs[0].set_xlabel('Author')
      axs[0].set_ylabel('Number of Tweets')
  
      axs[1].bar(range(len(likes)), likes)
      axs[1].set_title('Number of Likes per Tweet')
      axs[1].set_xlabel('Tweet Index')
      axs[1].set_ylabel('Number of Likes')
  
      axs[2].bar(range(len(retweets)), retweets)
      axs[2].set_title('Number of Retweets per Tweet')
      axs[2].set_xlabel('Tweet Index')
      axs[2].set_ylabel('Number of Retweets')
  
      plt.tight_layout()
      plt.show()