jupyter lab fixes (#1139) [skip ci]

* add a basic notebook for lab users in the root

* update notebook and fix cors for jupyter

* cell is code

* fix eval batch size check

* remove intro notebook

docker/Dockerfile-cloud

@@ -12,7 +12,7 @@ EXPOSE 22
 
 COPY scripts/cloud-entrypoint.sh /root/cloud-entrypoint.sh
 
-RUN pip install jupyterlab notebook && \
+RUN pip install jupyterlab notebook ipywidgets && \
     jupyter lab clean
 RUN apt install --yes --no-install-recommends openssh-server tmux && \
     mkdir -p ~/.ssh && \