class_docstr: Tokenizes by making a request to the proxy server with REST endpoint: `/api/tokenize`.
docstr: AI21 tokenizers do no have a prefix token
docstr_tok: ["AI21","tokenizers","do","no","have","a","prefix","token"]
code: def prefix_token(self) -> str:
      """AI21 tokenizers do no have a prefix token"""
      return ""
  
imports: import re
  from typing import List, Optional, Tuple
  from urllib.parse import unquote
  from helm.common.tokenization_request import TokenizationRequest, TokenizationRequestResult, TokenizationToken, TextRange
  from .window_service import WindowService, EncodeResult
  from .tokenizer_service import TokenizerService
  from .gpt2_window_service import GPT2WindowService
label_desc: Use image node_id 6 for calling the AI21WindowService obj's underlying member method code with example usage: obj.prefix_token() and returns: str