1167 fixes protobuf (#1171)

Fixes #1167

### Description
- pip install ray might upgrade protobuf automatically, but protobuf v4
is not compatible with some versions of pytorch's tensorboard
summarywriter

### 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

deployment/ray/mednist_classifier_ray.ipynb

@@ -41,7 +41,7 @@
    },
    "outputs": [],
    "source": [
-    "%pip install ray[serve]\n",
+    "!pip install \"ray[serve]\" \"protobuf<4\"\n",
     "!python -c \"import monai\" || pip install -q \"monai-weekly[gdown, tqdm]\""
    ]
   },

runner.sh

@@ -517,7 +517,7 @@ for file in "${files[@]}"; do
         then
             export PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python
         else
-            export PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=
+            unset PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION
         fi
 
         cmd=$(echo "papermill ${papermill_opt} --progress-bar -k ${kernelspec}")