Replaces #1502

The old pyro.util.save_visualization() has languished, but users seem to like numpyro.render_model. Let's port this functionality back to Pyro.

Tasks

• Support discrete latent variables in pyro.inspect.get_dependencies(). @ordabayevy created a great solution to this in funsor's ProvenanceTensor; it should be easy to refactor get_dependencies() to use this rather than autograd to determine dependency structure. Support discrete variables in get_dependencies() via ProvenanceTensor #2959
• Implement pyro.render_model() based on pyro.inspect.get_dependencies() Port numpyro.render_model() to Pyro #2962