You are trying to call each feature layer with the same input, i.e. x = torch.FloatTensor(8, 3, 256, 256).cuda(). That is why the second layer (BatchNorm with in_channels=64) is complaining.
If you want to call each layer one by one, you should reassign the output:
x = Variable(torch.FloatTensor(8, 3, 256, 256))
for feat in model.features:
     x = feat(x)