title: Gensim saved dictionary has no id2token
body: <p>I have saved a Gensim dictionary to disk. When I load it, the <code>id2token</code> attribute dict is not populated.</p>
  
  <p>A simple piece of the code that saves the dictionary:</p>
  
  <pre><code>dictionary = corpora.Dictionary(tag_docs)
  dictionary.save("tag_dictionary_lda.pkl")
  </code></pre>
  
  <p>Now when I load it (I'm loading it in an jupyter notebook), it still works fine for mapping tokens to IDs, but <code>id2token</code> does not work (I cannot map IDs to tokens) and in fact <code>id2token</code> is not populated at all.</p>
  
  <pre><code>&gt; dictionary = corpora.Dictionary.load("../data/tag_dictionary_lda.pkl")
  &gt; dictionary.token2id["love"]
  Out: 1613
  
  &gt; dictionary.doc2bow(["love"])
  Out: [(1613, 1)]
  
  &gt; dictionary.id2token[1613]
  Out: 
  ---------------------------------------------------------------------------
  KeyError                                  Traceback (most recent call last)
  &lt;ipython-input&gt; in &lt;module&gt;()
  ----&gt; 1 dictionary.id2token[1613]
  
  KeyError: 1613
  
  &gt; list(dictionary.id2token.keys())
  Out: []
  </code></pre>
  
  <p>Any thoughts? </p>