add installation of benchmark deps to release workflow (#385)

.github/workflows/release.yml

@@ -22,6 +22,7 @@ jobs:
         run: git submodule init && git submodule update && python3 -m venv ./venv && . venv/bin/activate
       - run: pip install --upgrade setuptools
       - run: pip install -r requirements.txt
+      - run: [ -e "requirements_benchmark.txt" ] && pip install -r requirements_benchmark.txt # include benchmark requirements if they exist.
       - run: pip install -e .
       - run: py.test