NFVI notebook improve (#2415)

* small improve of notebook

* fix typo
* bound potential function so NUTS can sample properly

* Add example for experiment.

* add notebook to docs

* remove bound

only bounding the logp is not ideal as NF will return inf. modified the potential function 2-4 following @ferrine 's suggestion

* resolved conflict

Author: Junpeng Lao

docs/source/examples.rst

@@ -74,3 +74,4 @@ Variational Inference
    notebooks/convolutional_vae_keras_advi.ipynb
    notebooks/bayesian_neural_network_opvi-advi.ipynb
    notebooks/empirical-approx-overvirew.ipynb
+   notebooks/normalizing_flows_overview.ipynb