code: def perplexity(self, sentences):
      inputs = self.tokenizer(sentences, return_tensors="pt")
      if len(inputs["input_ids"][0] > 1024):
          new_ids = inputs["input_ids"][0][:1024]
          new_mask = inputs["attention_mask"][0][:1024]
          inputs = {
              "input_ids": torch.stack([new_ids]),
              "attention_mask": torch.stack([new_mask]),
          }
  
      with torch.no_grad():
          outputs = self.model(
              **inputs, labels=inputs["input_ids"], return_dict=True
          )
      return math.exp(outputs.loss.item())
  
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 4 for calling the GPT2Perplexity obj's underlying member method code with example usage: obj.perplexity(sentences) and returns: math