Hi again…
What I tried to say was …
device = ("cuda" if torch.cuda.is_available() else "cpu" )
model = net()
if torch.cuda.device_count() > 1:
    # device_ids has a default : all
    model = torch.nn.DataParallel(model, device_ids=[0, 1, 2, 3]) 
model.to(device)