docstr: Calculate the score
  
  :param str transformed_text: transformed sentence
  :param str reference_text: origin sentence
  :return float: the score of two sentence
code: def validate(self, transformed_text, reference_text):
      r"""
      Calculate the score
  
      :param str transformed_text: transformed sentence
      :param str reference_text: origin sentence
      :return float: the score of two sentence
      """
      return min(
          self.perplexity(reference_text)
          / self.perplexity(transformed_text),
          1,
      )
  
file_path: textflint/textflint/generation/validator/gpt2_perplexity.py
imports: import math
  import torch
  from transformers import GPT2Tokenizer, GPT2LMHeadModel
  from .validator import Validator
label_desc: Use image node_id 3 for calling the GPT2Perplexity obj's underlying member method code with example usage: obj.validate(transformed_text, reference_text) and returns: min