text: Title: how to implemente get_document_topics()
  Body: Hi all,
  I use the method of AuthorTopicModel() build a model, then i want to get the topic of each document or the document belongs to which topic(assume that i have 10 topics) and the tutorial tell me that the method of get_document_topics() are not be implemented, so how to implemente it.
  
  thanks in advance
  yi zhao
  