integration test error

[wyli]

Describe the bug
TestIntegrationSlidingWindow error introduced by #2308
please see log (https://github.com/Project-MONAI/MONAI/runs/2753864977?check_suite_focus=true):

======================================================================
ERROR: test_training (__main__.TestIntegrationSlidingWindow)
----------------------------------------------------------------------
ValueError: too many values to unpack (expected 3)

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/__w/MONAI/MONAI/tests/utils.py", line 446, in _wrapper
    raise RuntimeError(res.traceback) from res
RuntimeError: Traceback (most recent call last):
  File "/__w/MONAI/MONAI/tests/utils.py", line 385, in run_process
    output = func(*args, **kwargs)
  File "/__w/MONAI/MONAI/tests/utils.py", line 471, in _call_original_func
    return f(*args, **kwargs)
  File "/__w/MONAI/MONAI/tests/test_integration_sliding_window.py", line 83, in test_training
    batch_size=2, img_name=self.img_name, seg_name=self.seg_name, output_dir=tempdir, device=self.device
  File "/__w/MONAI/MONAI/tests/test_integration_sliding_window.py", line 54, in run_test
    infer_engine.run(loader)
  File "/opt/conda/lib/python3.6/site-packages/ignite/engine/engine.py", line 702, in run
    return self._internal_run()
  File "/opt/conda/lib/python3.6/site-packages/ignite/engine/engine.py", line 775, in _internal_run
    self._handle_exception(e)
  File "/opt/conda/lib/python3.6/site-packages/ignite/engine/engine.py", line 469, in _handle_exception
    raise e
  File "/opt/conda/lib/python3.6/site-packages/ignite/engine/engine.py", line 745, in _internal_run
    time_taken = self._run_once_on_dataset()
  File "/opt/conda/lib/python3.6/site-packages/ignite/engine/engine.py", line 850, in _run_once_on_dataset
    self._handle_exception(e)
  File "/opt/conda/lib/python3.6/site-packages/ignite/engine/engine.py", line 469, in _handle_exception
    raise e
  File "/opt/conda/lib/python3.6/site-packages/ignite/engine/engine.py", line 833, in _run_once_on_dataset
    self.state.output = self._process_function(self, self.state.batch)
  File "/__w/MONAI/MONAI/tests/test_integration_sliding_window.py", line 43, in _sliding_window_processor
    img, seg, meta_data = batch
ValueError: too many values to unpack (expected 3)