code: def test_get_gradients(self):
      inputs = {
          "sentence": "I always write unit tests",
      }
  
      archive = load_archive(
          self.FIXTURES_ROOT
          / "basic_classifier"
          / "serialization"
          / "model.tar.gz"
      )
      predictor = Predictor.from_archive(archive)
  
      instance = predictor._json_to_instance(inputs)
      predictor._dataset_reader.apply_token_indexers(instance)
      outputs = predictor._model.forward_on_instance(instance)
      labeled_instances = predictor.predictions_to_labeled_instances(
          instance, outputs
      )
      for instance in labeled_instances:
          grads = predictor.get_gradients([instance])[0]
          assert "grad_input_1" in grads
          assert grads["grad_input_1"] is not None
          assert len(grads["grad_input_1"][0]) == 5
  
file_path: allennlp/tests/predictors/predictor_test.py
imports: from allennlp.common.testing import AllenNlpTestCase
  from allennlp.models.archival import load_archive
  from allennlp.predictors import Predictor
  from allennlp.nn import util
label_desc: Use image node_id 3 for calling the TestPredictor obj's underlying member method code with example usage: obj.test_get_gradients() without return types