[doc] fix device issue (#6660)

notebooks/graphbolt/walkthrough.ipynb

@@ -27,10 +27,9 @@
         "os.environ['TORCH'] = torch.__version__\n",
         "os.environ['DGLBACKEND'] = \"pytorch\"\n",
         "\n",
-        "# Uncomment below to install required packages. If the CUDA version is not 11.8,\n",
-        "# check the https://www.dgl.ai/pages/start.html to find the supported CUDA\n",
-        "# version and corresponding command to install DGL.\n",
-        "# !pip install --pre dgl -f https://data.dgl.ai/wheels-test/cu118/repo.html > /dev/null\n",
+        "# Install the CPU version.\n",
+        "device = torch.device(\"cpu\")\n",
+        "!pip install --pre dgl -f https://data.dgl.ai/wheels-test/repo.html\n",
         "\n",
         "try:\n",
         "    import dgl.graphbolt as gb\n",
@@ -259,7 +258,7 @@
       },
       "outputs": [],
       "source": [
-        "datapipe = datapipe.copy_to(device=\"cuda\")\n",
+        "datapipe = datapipe.copy_to(device=device)\n",
         "print(next(iter(datapipe)))"
       ]
     },
@@ -318,7 +317,6 @@
   "metadata": {
     "accelerator": "GPU",
     "colab": {
-      "authorship_tag": "ABX9TyOCdFtYQweXnIR1/5oWDSGq",
       "collapsed_sections": [
         "BjkAK37xopp1"
       ],