some clean up

app.py

@@ -58,7 +58,6 @@ def generate_adversial_image(img, epsilon):
     # compute gradient
     label = torch.Tensor([top5_idx[0]]).type(torch.LongTensor)
     loss = torch.nn.functional.cross_entropy(output, label)
-    model.zero_grad()
     loss.backward()
     
     # generate adversarial image