[Fix] multiprocessing bug with pickle

[mbml84]

[mbml84]

Fix a pickle bug when using multiprocessing

[Gabri95]

Hi @mbml84

Thanks for the pull request!

I am actually having a similar issue in the new escnn library.

Your code should fix the issue here but it seems to involve some large changes in the backend, so I think I need to take some extra care to integrate them in the code base.

Because I am already working on this in the new version of the library and I am trying to discontinue e2cnn, I would point you to escnn. Your pull request seems useful and I'd like it if you opened a similar one there (although I understand that might be some work for you, sorry).

If, for compatibility issues, more people need this feature in e2cnn, I will try to fix the issue here too.

Thanks again, Gabriele