[Tutorial] Add a small test for GAT exercise (#5644)

Co-authored-by: Hongzhi (Steve), Chen <chenhongzhi.nkcs@gmail.com>

notebooks/sparse/quickstart.ipynb

@@ -1307,6 +1307,21 @@
       },
       "execution_count": null,
       "outputs": []
+    },
+    {
+      "cell_type": "code",
+      "source": [
+        "# Test:\n",
+        "# Let's use the symmetric A created above.\n",
+        "X = torch.randn(6, 20)\n",
+        "module = SimplifiedGAT(20, 10)\n",
+        "Y = module(A, X)"
+      ],
+      "metadata": {
+        "id": "qjcXiidYCqGK"
+      },
+      "execution_count": null,
+      "outputs": []
     }
   ]
 }