Fix backward compatibilty

## 🐛 Bug description

Recent PR on the change of `mp.spawn` by `mp.start_process` brakes compatibility with PyTorch 1.3.1 and 1.4.0 versions.

https://github.com/pytorch/ignite/actions?query=workflow%3A.github%2Fworkflows%2Fpytorch-version-tests.yml

We have to fix that.