code: def test_exotic_tokens_no_nan_grads(self):
      token_indexer = PretrainedTransformerMismatchedIndexer(
          "bert-base-uncased"
      )
  
      sentence1 = ["A", "", "AllenNLP", "sentence", "."]
      sentence2 = [
          "A",
          "\uf732\uf730\uf730\uf733",
          "AllenNLP",
          "sentence",
          ".",
      ]
  
      tokens1 = [Token(word) for word in sentence1]
      tokens2 = [Token(word) for word in sentence2]
      vocab = Vocabulary()
  
      token_embedder = BasicTextFieldEmbedder(
          {
              "bert": PretrainedTransformerMismatchedEmbedder(
                  "bert-base-uncased"
              )
          }
      )
  
      instance1 = Instance(
          {"tokens": TextField(tokens1, {"bert": token_indexer})}
      )
      instance2 = Instance(
          {"tokens": TextField(tokens2, {"bert": token_indexer})}
      )
  
      batch = Batch([instance1, instance2])
      batch.index_instances(vocab)
  
      padding_lengths = batch.get_padding_lengths()
      tensor_dict = batch.as_tensor_dict(padding_lengths)
      tokens = tensor_dict["tokens"]
  
      bert_vectors = token_embedder(tokens)
      test_loss = bert_vectors.mean()
  
      test_loss.backward()
  
      for name, param in token_embedder.named_parameters():
          grad = param.grad
          assert (grad is None) or (
              not torch.any(torch.isnan(grad)).item()
          )
  
imports: import pytest
  import torch
  from allennlp.common import Params, cached_transformers
  from allennlp.common.checks import ConfigurationError
  from allennlp.data import Token, Vocabulary
  from allennlp.data.batch import Batch
  from allennlp.data.fields import TextField
  from allennlp.data.instance import Instance
  from allennlp.data.token_indexers import PretrainedTransformerMismatchedIndexer
  from allennlp.modules.text_field_embedders import BasicTextFieldEmbedder
  from allennlp.modules.token_embedders import PretrainedTransformerMismatchedEmbedder
  from allennlp.common.testing import AllenNlpTestCase
label_desc: Use image node_id 5 for calling the TestPretrainedTransformerMismatchedEmbedder obj's underlying member method code with example usage: obj.test_exotic_tokens_no_nan_grads() without return types