class_lung_lesion.ipynb testing error

[wyli]

Describe the bug
from the ci tests because of Project-MONAI/MONAI#2137:

Running ./modules/interpretability/class_lung_lesion.ipynb
Checking PEP8 compliance...
Running notebook...
Before:
    "max_epochs = 100\n",
After:
    "max_epochs = 1\n",
Before:
    "val_interval = 1\n",
After:
    "val_interval = 1\n",

Executing:   0%|          | 0/20 [00:00<?, ?cell/s]
Executing:   5%|▌         | 1/20 [00:00<00:18,  1.02cell/s]
Executing:  10%|█         | 2/20 [00:04<00:32,  1.79s/cell]
Executing:  15%|█▌        | 3/20 [00:07<00:34,  2.02s/cell]
Executing:  20%|██        | 4/20 [00:19<01:22,  5.13s/cell]
Executing:  45%|████▌     | 9/20 [00:19<00:39,  3.61s/cell]
Executing:  45%|████▌     | 9/20 [00:21<00:25,  2.36s/cell]
Traceback (most recent call last):
  File "/opt/conda/bin/papermill", line 8, in <module>
    sys.exit(papermill())
  File "/opt/conda/lib/python3.8/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/opt/conda/lib/python3.8/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/opt/conda/lib/python3.8/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/opt/conda/lib/python3.8/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/opt/conda/lib/python3.8/site-packages/click/decorators.py", line 21, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/opt/conda/lib/python3.8/site-packages/papermill/cli.py", line 250, in papermill
    execute_notebook(
  File "/opt/conda/lib/python3.8/site-packages/papermill/execute.py", line 122, in execute_notebook
    raise_for_execution_errors(nb, output_path)
  File "/opt/conda/lib/python3.8/site-packages/papermill/execute.py", line 234, in raise_for_execution_errors
    raise error
papermill.exceptions.PapermillExecutionError: 
---------------------------------------------------------------------------
Exception encountered at "In [6]":
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-6-8592e0c07ebc> in <module>
     39 )
     40 
---> 41 train_ds = monai.data.PersistentDataset(
     42     data=train_files, transform=train_transforms
     43 )

TypeError: __init__() missing 1 required positional argument: 'cache_dir'