text: Title: visualization of only 3 layers / example model_view_xlnet.ipynb
  Body: I tried load XLNet only with three layers (it does work with full XLNet) but with three the example model_view_xlnet.ipynb does not work
  
  ```
  config = XLNetConfig.from_pretrained('/transformers/')
  config.n_layer = 3
  config.num_labels = 3
  model = XLNetModel.from_pretrained('/transformers/')
  ```
  
  
  ```
  ---------------------------------------------------------------------------
  ValueError                                Traceback (most recent call last)
  <ipython-input-11-7c9c3356caa4> in <module>
       17 input_id_list = input_ids[0].tolist() # Batch index 0
       18 tokens = tokenizer.convert_ids_to_tokens(input_id_list)
  ---> 19 model_view(attention, tokens)
  
  ~/projects/bertviz/bertviz/model_view.py in model_view(attention, tokens, sentence_b_start, prettify_tokens)
       78     attn_seq_len = len(attn_data['all']['attn'][0][0])
       79     if attn_seq_len != len(tokens):
  ---> 80         raise ValueError(f"Attention has {attn_seq_len} positions, while number of tokens is {len(tokens)}")
       81     display(Javascript('window.params = %s' % json.dumps(params)))
       82     display(Javascript(vis_js))
  
  ValueError: Attention has 768 positions, while number of tokens is 14
  ```