class_docstr: This class handles the parser event from opt_parser.
  When a target statement is matched, an event is triggered,
  we collected necessary information from this event and construct
  the branches as what we want to understand.
code: def token(self):
      if self.token_list:
          tok = self.token_list.pop(0)
          return tok
      return None
  
imports: import nnabla
label_desc: Use image node_id 5 for calling the OptimizerBuilder obj's underlying member method code with example usage: obj.token() and returns: None, tok