Dask PubSub is now deprecated

After Dask release 2025.4.0, it is not possible to use the Pub/Sub system in Dask.

This is not available anymore. This is the main premise of the dask-pytorch-ddp project.

The reason why I did not open a PR is that I don't know yet the right replacement for it.

For further reference, see https://github.com/dask/dask/releases/tag/2025.4.0