[BUG] CGNN (Causal Graph Generation) + Usage of multiprocessing with pytorch

[AmanTiwari1503]

Describe the bug
Usage of the multiprocessing library along with torch for multiple processes leads to the following error

p = cdt.causality.graph.CGNN().predict(df)
An exhaustive search of the causal structure of CGNN without skeleton is super-exponential in the number of variables.
A total of 3 graphs will be evaluated.
Process Process-3:
Process Process-2:
Traceback (most recent call last):
Traceback (most recent call last):
File "/path/to/python/installation/multiprocessing/process.py", line 297, in _bootstrap
self.run()
File "/path/to/python/installation/multiprocessing/process.py", line 297, in _bootstrap
self.run()
File "/path/to/python/installation/multiprocessing/process.py", line 99, in run
self._target(*self._args, **self._kwargs)
File "/path/to/python/installation/multiprocessing/process.py", line 99, in run
self._target(*self._args, **self._kwargs)
File "/path/to/python/installation/site-packages/cdt/utils/parallel.py", line 54, in worker_subprocess
output = function(*args, **kwargs, device=device, idx=idx)
File "/path/to/python/installation/site-packages/cdt/utils/parallel.py", line 54, in worker_subprocess
output = function(*args, **kwargs, device=device, idx=idx)
File "/path/to/python/installation/site-packages/cdt/causality/graph/CGNN.py", line 197, in graph_evaluation
obs = th.Tensor(scale(data.values)).to(device)
File "/path/to/python/installation/site-packages/cdt/causality/graph/CGNN.py", line 197, in graph_evaluation
obs = th.Tensor(scale(data.values)).to(device)
File "/path/to/python/installation/site-packages/torch/cuda/init.py", line 208, in _lazy_init
"Cannot re-initialize CUDA in forked subprocess. To use CUDA with "
File "/path/to/python/installation/site-packages/torch/cuda/init.py", line 208, in _lazy_init
"Cannot re-initialize CUDA in forked subprocess. To use CUDA with "
RuntimeError: Cannot re-initialize CUDA in forked subprocess. To use CUDA with multiprocessing, you must use the 'spawn' start method
RuntimeError: Cannot re-initialize CUDA in forked subprocess. To use CUDA with multiprocessing, you must use the 'spawn' start method

Possibly because of the parallel_run function in the cdt/utils/parallel.py file

Please mention
- cdt version - 0.5.23
- Python version - 3.7.9
- PyTorch package version -1.12.0+cu102
- GPU used - Tesla V100 PCIe 16GB

Possible fixes
Please refer to this stack overflow solution - https://stackoverflow.com/questions/48822463/how-to-use-pytorch-multiprocessing

[diviyank]

Hello !
You're right, I thought I solved this issue though... I'll check