Fix memory leak

Chapter 3/minimal_vlm_training.ipynb

@@ -462,7 +462,7 @@
     "\n",
     "    optimizer.zero_grad()\n",
     "    logits, loss = vlm(text, images)\n",
-    "    losses.append(loss)\n",
+    "    losses.append(loss.item())\n",
     "\n",
     "    loss.backward()\n",
     "    optimizer.step()\n",
@@ -471,7 +471,7 @@
     "        print(f\"step {step} | loss {loss.item():.4f}\")\n",
     "\n",
     "    if step > 5000:\n",
-    "      json.dump([round(l.item(), 4) for l in losses], open(\"chapter_3_loss.json\", \"w\"))\n",
+    "      json.dump([round(l, 4) for l in losses], open(\"chapter_3_loss.json\", \"w\"))\n",
     "      break"
    ]
   },