update d2go_beginner.ipynb for exporting model

Summary:
Pull Request resolved: #155

- remove tracing, which shouldn't affect anything.
- create the model in cpu mode, since it might have issue casting model.

Reviewed By: zhanghang1989

Differential Revision: D33357269

fbshipit-source-id: 27a0330ebb12b993744dee47151c3056cd584ccf

demo/d2go_beginner.ipynb

@@ -592,14 +592,14 @@
     "patch_d2_meta_arch()\n",
     "\n",
     "cfg_name = 'faster_rcnn_fbnetv3a_dsmask_C4.yaml'\n",
-    "pytorch_model = model_zoo.get(cfg_name, trained=True)\n",
-    "pytorch_model.cpu()\n",
+    "pytorch_model = model_zoo.get(cfg_name, trained=True, device='cpu')\n",
+    "pytorch_model.eval()\n",
     "\n",
     "with create_fake_detection_data_loader(224, 320, is_train=False) as data_loader:\n",
     "    predictor_path = convert_and_export_predictor(\n",
     "            model_zoo.get_config(cfg_name),\n",
     "            copy.deepcopy(pytorch_model),\n",
-    "            \"torchscript_int8@tracing\",\n",
+    "            \"torchscript_int8\",\n",
     "            './',\n",
     "            data_loader,\n",
     "        )\n",