Add classic hierarchical Bayesian model example

[eb8680]

Paper: Hierarchical Models for Causal Effects

[eb8680]

@qinqian this or #18 might be good starting points for contribution. This one in particular is reasonably self-contained, and there are already a few related examples in Pyro and NumPyro, e.g. here.

[qinqian]

sure, thanks for the advice @eb8680! I'll review this and #18 to see how to add them to the examples of the tutorial.