title: How to save gensim doc2vec model
body: <p>after train the model, I use  infer_vector() to get the vector successfully.
  but after I save the model and load again, error appears as follows:</p>
  
  <pre><code>print "infer:", model.infer_vector(sents[0]).tolist()
  File "/Users/zhangweimin/anaconda/lib/python2.7/site-packages/gensim/models/doc2vec.py", line 752, in infer_vector
      doctag_vectors=doctag_vectors, doctag_locks=doctag_locks)
  File "gensim/models/doc2vec_inner.pyx", line 426, in gensim.models.doc2vec_inner.train_document_dm (./gensim/models/doc2vec_inner.c:5401)
  TypeError: object of type 'NoneType' has no len()
  </code></pre>
  
  <p>the whole code is:`    </p>
  
  <pre><code>model = train_d2v(labeled_docs, model_file, word_file, 3)
  
  # OK
  print "before infer:", model.infer_vector(sents[0]).tolist()
  
  model = Doc2Vec.load(model_file)
  
  print "sents[0]:", sents[0]
  print "type:", type(model)
  print "infer:", model.infer_vector(sents[0]).tolist() #ERROR`
  </code></pre>