docstr: Set special tokens according to bert tokenizer.
docstr_tok: ["Set","special","tokens","according","to","bert","tokenizer","."]
code: def override_special_tokens(self, opt: Opt):
      """
      Set special tokens according to bert tokenizer.
      """
      # now override
      self.start_token = self.hf_tokenizer.cls_token
      self.end_token = self.hf_tokenizer.sep_token
      self.null_token = self.hf_tokenizer.pad_token
      self.unk_token = self.hf_tokenizer.unk_token
  
      self._unk_token_idx = self.hf_tokenizer.unk_token_id
  
      self.start_idx = self[self.start_token]
      self.end_idx = self[self.end_token]
      self.null_idx = self[self.null_token]
  
imports: from parlai.agents.transformer.transformer import TransformerRankerAgent
  from parlai.agents.transformer.modules import TransformerMemNetModel, TransformerEncoder
  from parlai.agents.hugging_face.dict import HuggingFaceDictionaryAgent
  from parlai.agents.rag.args import DPR_ZOO_MODEL
  from parlai.agents.rag.conversion_utils import BertConversionUtils
  from parlai.core.build_data import modelzoo_path
  from parlai.core.dict import DictionaryAgent
  from parlai.core.loader import register_agent
  from parlai.core.opt import Opt
  from parlai.core.torch_ranker_agent import TorchRankerAgent
  from parlai.utils.io import PathManager
  import parlai.utils.logging
  import os
  import torch
  import torch.cuda
  import torch.nn
  from transformers import BertConfig
  from typing import Dict, Optional
label_desc: Use image node_id 4 for calling the BertTokenizerDictionaryAgent obj's underlying member method code with example usage: obj.override_special_tokens(opt) without return types