I created a simpler version of my code. If you run this code you will find the unusual worker processes to be spawned on GPUs. I have found out that these processes are spawned only if I set a custom  collate_fn into my DataLoader. If you use the default collate_fn the strange processes do not seem to spawn. It is not yet clear to me the why.
code: PyTorch DDP memory inspection - Pastebin.com 6