tracegraph_elbo related cleanup

- [x] refactor vectorized_map_data_info and related data structures
- [x] refactor way prune_subsample_sites etc. is used to setup a trace for computation
- [x] rethink any needed asserts/warnings in tracegraph_elbo or trace_poutine
- [x] go through test_valid_models and make sure we have good coverage
- [x] rethink integration/gradient test coverage (where possibly prefer cheaper tests)
