code: def test_forward(self, mock_forward_train, mock_forward_test):
      def mock_func_train(feat, out_enc, datasamples):
          return True
  
      def mock_func_test(feat, out_enc, datasamples):
          return False
  
      tmp_dir = tempfile.TemporaryDirectory()
      dict_file = osp.join(tmp_dir.name, "fake_chars.txt")
      create_dummy_dict_file(dict_file)
      dict_cfg = dict(
          type="Dictionary",
          dict_file=dict_file,
          with_start=True,
          with_end=True,
          same_start_end=False,
          with_padding=True,
          with_unknown=True,
      )
      mock_forward_train.side_effect = mock_func_train
      mock_forward_test.side_effect = mock_func_test
      cfg = dict(type="Tmp")
      decoder = BaseDecoder(dict_cfg, cfg, cfg)
      # test loss
      loss = decoder.loss(None, None, None)
      self.assertIsNone(loss)
  
      # test predict
      predict = decoder.predict(None, None, None)
      self.assertIsNone(predict)
  
      # test forward
      tensor = decoder(None, None, None)
      self.assertTrue(tensor)
      decoder.eval()
      tensor = decoder(None, None, None)
      self.assertFalse(tensor)
      tmp_dir.cleanup()
  
file_path: mmocr/tests/test_models/test_textrecog/test_decoders/test_base_decoder.py
imports: import os.path
  import tempfile
  from unittest import TestCase, mock
  from mmocr.models.common.dictionary import Dictionary
  from mmocr.models.textrecog.decoders import BaseDecoder
  from mmocr.registry import MODELS, TASK_UTILS
  from mmocr.testing import create_dummy_dict_file
label_desc: Use image node_id 4 for calling the TestBaseDecoder obj's underlying member method code with example usage: obj.test_forward(mock_forward_train, mock_forward_test) and returns: True, False