title: IndexError: index out of range in self while try to fine tune Roberta model after adding special tokens
body: <p>I am trying to fine tune a Roberta model after adding some special tokens to its tokenizer:</p>
  <pre><code>    special_tokens_dict = {'additional_special_tokens': ['[Tok1]','[Tok2]']}
  
      tokenizer.add_special_tokens(special_tokens_dict)
  </code></pre>
  <p>I get this error when i try to train the model (on cpu):</p>
  <pre><code>IndexError                                Traceback (most recent call last)
  &lt;ipython-input-75-d63f8d3c6c67&gt; in &lt;module&gt;()
       50         l = model(b_input_ids, 
       51                      attention_mask=b_input_mask,
  ---&gt; 52                     labels=b_labels)
       53         loss,logits = l
       54         total_train_loss += l[0].item()
  
  8 frames
  /usr/local/lib/python3.6/dist-packages/torch/nn/functional.py in embedding(input, weight, padding_idx, max_norm, norm_type, scale_grad_by_freq, sparse)
     1850         # remove once script supports set_grad_enabled
     1851         _no_grad_embedding_renorm_(weight, input, max_norm, norm_type)
  -&gt; 1852     return torch.embedding(weight, input, padding_idx, scale_grad_by_freq, sparse)
     1853 
     1854 
  
  IndexError: index out of range in self
  </code></pre>
  <p>p.s. If I comment <code>add_special_tokens</code> the code works.</p>