Simplify TraceGraph_ELBO.loss_and_grads() (#536)
Simplify TraceGraph_ELBO.loss_and_grads() (#536)

* Factor out _loss_and_grads_particle() from loss_and_grads()

* Simplify _loss_and_grads_particle()

* Simplify TraceGraph_ELBO._loss_and_grads_particle()

uber/pyro,uber/pyro,uber/pyro