title: Tokenizer didn't add BOS token when encoding the sentence
body: <p>I would like to encode the sentence with BOS and EOS token. When I load a pretrained tokenizer, there is no BOS token, so I added BOS token to the tokenizer. After that, I encoded the sentence.</p>
  <pre><code>model_checkpoint = &quot;facebook/wmt19-en-de&quot;
  
  tokenizer = AutoTokenizer.from_pretrained(model_checkpoint)
  tokenizer.add_special_tokens({'bos_token' : '&lt;s&gt;'})
  
  tokenizer.encode(&quot;Resumption of the session&quot;, add_special_tokens = True)
  
  result: [2642, 4584, 636, 9, 6, 9485, 2] # 2642 is not BOS token, and 2 is EOS token.
  </code></pre>
  <p>However, the result shows that BOS token does not appear in the encoded sentence. How could I include BOS token when encoding?</p>