fork vs spawn on MacOS Python 3.9 error

Hi all,

I'm running macos conda-forge on the M1 architecture and testing the mednist_tutorial.ipynb and other jupyter notebooks. I get the following error

  File "/opt/homebrew/Caskroom/miniforge/base/envs/pytorch_env/lib/python3.9/multiprocessing/spawn.py", line 116, in spawn_main
    exitcode = _main(fd, parent_sentinel)
  File "/opt/homebrew/Caskroom/miniforge/base/envs/pytorch_env/lib/python3.9/multiprocessing/spawn.py", line 126, in _main
    self = reduction.pickle.load(from_parent)
AttributeError: Can't get attribute 'MedNISTDataset' on <module '__main__' (built-in)>

This seems to be caused by spawn which apparently became default by Python 3.8 and later. It is fixed by adding the argument multiprocessing_context="fork" to calls of torch.utils.data.DataLoader, however, I suggest that a deeper fixed is made for non-experts.

Steps to reproduce the behavior
On MacOS/python-forge
2. jupyter lab mednist_tutorial.ipynb
3. press the clear kernel and run all button

Expected behavior
A demo network should be set up and trained

Environment

% python --version
Python 3.9.10
% python -c 'import monai; monai.config.print_debug_info()'

================================
Printing MONAI config...
================================
MONAI version: 0.9.dev2210
Numpy version: 1.22.3
Pytorch version: 1.10.2
MONAI flags: HAS_EXT = False, USE_COMPILED = False
MONAI rev id: 1a660e6a7a50e985af5ff76b559baab44175438c
MONAI __file__: /opt/homebrew/Caskroom/miniforge/base/envs/pytorch_env/lib/python3.9/site-packages/monai/__init__.py

Optional dependencies:
Pytorch Ignite version: NOT INSTALLED or UNKNOWN VERSION.
Nibabel version: NOT INSTALLED or UNKNOWN VERSION.
scikit-image version: NOT INSTALLED or UNKNOWN VERSION.
Pillow version: 9.0.1
Tensorboard version: 2.8.0
gdown version: NOT INSTALLED or UNKNOWN VERSION.
TorchVision version: NOT INSTALLED or UNKNOWN VERSION.
tqdm version: 4.63.0
lmdb version: NOT INSTALLED or UNKNOWN VERSION.
psutil version: NOT INSTALLED or UNKNOWN VERSION.
pandas version: NOT INSTALLED or UNKNOWN VERSION.
einops version: 0.4.1
transformers version: NOT INSTALLED or UNKNOWN VERSION.
mlflow version: NOT INSTALLED or UNKNOWN VERSION.

For details about installing the optional dependencies, please visit:
    https://docs.monai.io/en/latest/installation.html#installing-the-recommended-dependencies


================================
Printing system config...
================================
`psutil` required for `print_system_info`

================================
Printing GPU config...
================================
Num GPUs: 0
Has CUDA: False
cuDNN enabled: False

Thank you for a nice package, Jon

macOS 上の Python 3.9 を使用して Jupyter Lab で mednist_tutorial.ipynb から始め、カーネルをクリアしてすべてを実行することでエラーを再現し、その後ノートブック内の torch.utils.data.DataLoader の呼び出しを調べます。MedNISTDataset の multiprocessing エラーなしでデモネットワークをセットアップしてトレーニングできれば完了です。