text: Title: Add LDA example to illustrate some usage of `flax_module`/`haiku_module`
  Body: After #1016 and #989, it would be nice to port [ProdLDA](http://pyro.ai/examples/prodlda.html#ProdLDA:-Latent-Dirichlet-Allocation-with-Product-of-Experts) example in Pyro to illustrate some usage of JAX nn frameworks `flax`/`haiku` in NumPyro. We might consider replacing `SoftmaxNormal` there with `Dirichlet` distribution. This is a good first issue for those who are interested in both NLP and Bayesian inference.