input: Write a function to visually summarize the Twitter data by processing the stream of tweets and plotting the author activity, number of likes, and number of retweets using matplotlib. The function should take in the Twitter data as input and generate the required visualizations.
  
  import matplotlib.pyplot as plt
  
  def visualize_twitter_data(twitter_data):
      # Process the Twitter data to extract
      # the relevant data fields
      authors = []
      num_tweets = []
      num_likes = []
      num_retweets = []
      # ...
  
      # Plot the tweet authors
      plt.xlabel('Tweet Authors')
      plt.ylabel('Number of tweets')
      plt.title('Tweet Author Summary')
      plt.bar(authors, num_tweets)
      plt.show()
  
      # Plot the number of likes
      plt.xlabel('Tweets')
      plt.ylabel('Number of Likes')
      plt.title('Number of Likes Summary')
      plt.bar(tweets, num_likes)
      plt.show()
  
      # Plot the number of retweets
      plt.xlabel('Tweets')
      plt.ylabel('Number of Retweets')
      plt.title('Number of Retweets Summary')
      plt.bar(tweets, num_retweets)
      plt.show()
  
  assert visualize_twitter_data([{'author': 'User1', 'likes': 5, 'retweets': 2}, {'author': 'User2', 'likes': 10, 'retweets': 4}]) == None
  assert visualize_twitter_data([{'author': 'User1', 'likes': 3, 'retweets': 1}, {'author': 'User1', 'likes': 7, 'retweets': 3}]) == None
  assert visualize_twitter_data([]) == None
output: def visualize_twitter_data(twitter_data):
      authors = []
      num_tweets = []
      num_likes = []
      num_retweets = []
      
      for tweet in twitter_data:
          author = tweet['author']
          likes = tweet['likes']
          retweets = tweet['retweets']
          
          if author in authors:
              index = authors.index(author)
              num_tweets[index] += 1
              num_likes[index] += likes
              num_retweets[index] += retweets
          else:
              authors.append(author)
              num_tweets.append(1)
              num_likes.append(likes)
              num_retweets.append(retweets)
      
      plt.xlabel('Tweet Authors')
      plt.ylabel('Number of tweets')
      plt.title('Tweet Author Summary')
      plt.bar(authors, num_tweets)
      plt.show()
  
      plt.xlabel('Tweets')
      plt.ylabel('Number of Likes')
      plt.title('Number of Likes Summary')
      plt.bar(authors, num_likes)
      plt.show()
  
      plt.xlabel('Tweets')
      plt.ylabel('Number of Retweets')
      plt.title('Number of Retweets Summary')
      plt.bar(authors, num_retweets)
      plt.show()