Port poutine.collapse to NumPyro

[fritzo]

Pyro introduced a poutine.collapse() effect in pyro-ppl/pyro#2620, allowing exact conjugate inference over blocks of code, similar to @neerajprad's pyro.contrib.conjugate.

@fehiepsi I think this would be an edifying intro to using Funsor for general pattern matching. Once poutine.collapse() is implemented in NumPyro, we can add new conjugacy patterns to Funsor. Currently I believe only Beta-Binomial and Gaussian-Gaussian are implemented. I think some good starter Funsor tasks would be implementing patterns for say Beta-Bernoulli and then patterns for plates, as in Beta-plated(Binomial).

cc @eb8680

[fehiepsi]

Thanks, @fritzo! Let me assign this to myself. And will do some conjugate stuffs after this.