class_docstr: Convert between text-label and text-index 
class_docstr_tok: ["Convert","between","text-label","and","text-index"]
code: def add_special_char(self, dict_character):
      dict_character = ["blank"] + dict_character
      return dict_character
  
imports: import numpy
  import paddle
  from paddle.nn import functional
  import re
  from shapely.geometry import Polygon
  import cv2
  import copy
label_desc: Use image node_id 3 for calling the CTCLabelDecode obj's underlying member method code with example usage: obj.add_special_char(dict_character) and returns: dict_character