input: Write a function that takes a stream of Twitter data as input, where the data fields include tweet author, date of tweet, text, number of likes, and number of retweets. The function should process the data to extract relevant fields and then generate visual summaries of the data using matplotlib. The function should be named `visualize_twitter_data` and should return the matplotlib plots.
  
  
  assert visualize_twitter_data([
      {"author": "Alice", "date": "2023-01-01", "text": "Hello", "likes": 10, "retweets": 5},
      {"author": "Bob", "date": "2023-01-02", "text": "World", "likes": 15, "retweets": 3},
      {"author": "Alice", "date": "2023-01-03", "text": "Python", "likes": 20, "retweets": 7}
  ])==True
  
output: import matplotlib.pyplot as plt
  
  def visualize_twitter_data(data):
      authors = [tweet['author'] for tweet in data]
      likes = [tweet['likes'] for tweet in data]
      retweets = [tweet['retweets'] for tweet in data]
      
      fig, ax = plt.subplots(2, 1, figsize=(10, 8))
      
      ax[0].bar(authors, likes, color='blue')
      ax[0].set_title('Likes by Author')
      ax[0].set_ylabel('Number of Likes')
      
      ax[1].bar(authors, retweets, color='green')
      ax[1].set_title('Retweets by Author')
      ax[1].set_ylabel('Number of Retweets')
      
      plt.tight_layout()
      plt.show()
      
      return True