1356 debug tensorboard cell (#1362)

Fixes #1356


### Checks
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [ ] Avoid including large-size files in the PR.
- [ ] Clean up long text outputs from code cells in the notebook.
- [ ] For security purposes, please check the contents and remove any
sensitive info such as user names and private key.
- [ ] Ensure (1) hyperlinks and markdown anchors are working (2) use
relative paths for tutorial repo files (3) put figure and graphs in the
`./figure` folder
- [ ] Notebook runs automatically `./runner.sh -t <path to .ipynb file>`

Signed-off-by: Wenqi Li <wenqil@nvidia.com>

Author: wyli

3d_segmentation/spleen_segmentation_3d_lightning.ipynb

@@ -465,7 +465,9 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "## View training in tensorboard"
+    "## View training in tensorboard\n",
+    "\n",
+    "Please uncomment the following cell to load tensorboard results."
    ]
   },
   {
@@ -515,8 +517,8 @@
     }
    ],
    "source": [
-    "%load_ext tensorboard\n",
-    "%tensorboard --logdir=$log_dir"
+    "# %load_ext tensorboard\n",
+    "# %tensorboard --logdir=$log_dir"
    ]
   },
   {