This looks fine on my machine:
CPU:
model = VGGNet()
x = Variable(torch.randn(1, 3, 224, 224))
output = model(x)
o1 = output.clone()
output = model(x)
o2 = output.clone()
(o1 - o2).abs().sum()
>> Variable containing:
 0
[torch.FloatTensor of size 1]

GPU:
...
(o1 - o2).abs().sum()
>> Variable containing:
1.00000e-06 *
  7.5102
[torch.cuda.FloatTensor of size 1 (GPU 0)]