In PyTorch >= 0.4.0 you don’t need Variables anymore, so you can safely remove it.
Anyway, your code runs fine:
model = Network()
model.cuda()

image1 = np.random.randn(1, 3, 28, 28).astype(np.float32)
image2 = np.random.randn(1, 3, 28, 28).astype(np.float32)

input_image = np.vstack([image1, image2])
input_image = torch.from_numpy(input_image).cuda()

output = model(input_image)

Do you get an error using my code snippet?