update for new version of torch

tutorial-contents-notebooks/306_optimizer.ipynb

@@ -249,7 +249,7 @@
     "            opt.zero_grad()                # clear gradients for next train\n",
     "            loss.backward()                # backpropagation, compute gradients\n",
     "            opt.step()                     # apply gradients\n",
-    "            l_his.append(loss.data[0])     # loss recoder\n",
+    "            l_his.append(loss.item())     # loss recoder\n",
     "\n",
     "labels = ['SGD', 'Momentum', 'RMSprop', 'Adam']\n",
     "for i, l_his in enumerate(losses_his):\n",