Fix logpdf_grad errors in @dist DSL.

[ztangent]

This PR addresses #496, including some of the indexing errors identified in that issue, but also by generalizing the backpropagation of gradients from the base distribution to the user-facing / custom-defined @dist distribution. Generalizing the backprop code was necessary to ensure that vector-valued arguments have appropriate gradients returned (e.g. the probability vector for a relabeled categorical), but also provides support for multivariate distributions (e.g. broadcasted_normal and mvnormal). Test cases have been added to check for this functionality.

The one additional test case I can think of is to make sure using a @dist-defined distribution in a generative function leads to the correct outputs (gradients, score, etc.). I can do this later today!

[alex-lew]

This looks great, @ztangent! Thanks so much.