add description of loading a pretrained bundle (#968)

Signed-off-by: binliu <binliu@nvidia.com>

### Description
Add codes of how to load a pretrained bundlein this tutorial.

### Checks
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- [ ] Notebook runs automatically `./runner [-p <regex_pattern>]`

Signed-off-by: binliu <binliu@nvidia.com>

Author: binliunls

computer_assisted_intervention/video_seg.ipynb

@@ -369,7 +369,14 @@
    "source": [
     "## Normal training loop\n",
     "\n",
-    "A normal training loop will be excuted. Meanwhile, the iou value of each epoch will be recorded as the accuracy."
+    "A normal training loop will be excuted. Meanwhile, the iou value of each epoch will be recorded as the accuracy. \n",
+    "There is a pretrained bundle for surgical tool segmentation task in the MONAI model-zoo, the model can be automatically downloaded and loaded to the network by adding codes to the block below:\n",
+    "```\n",
+    "pretrained_weights = monai.bundle.load(\n",
+    "    name=\"surgical_tool_segmentation\", bundle_dir=\"./\", version=\"0.2.0\"\n",
+    ")\n",
+    "model.load_state_dict(pretrained_weights)\n",
+    "```"
    ]
   },
   {