I tried the below snippet which is similar to yours but it works with multi-gpu on my device.
import torch.nn as nn

model = nn.Conv2d(3,3,1,1)
model = nn.DataParallel(model).cuda()

input = torch.randn(64,3,8,8).cuda()
output = model(input)

I’m not sure what the difference between mine and yours could be.