FR Return deterministics in AutoGuide.quantiles() by default

Currently, nodes that are wrapped in determinstic are not traced, and obtaining inferences from these nodes must be run through a Predictive object. When a user deliberately creates these deterministics they are signalling that their values are of central interest, so it would be convenient to provide them immediately and not force users to execute additional steps.

[fonnesbeck]

I will try and put together a toy example when I get a chance, but I am using the quantiles method on AutoDiagonalNormal in my particular case. This does not include any deterministics.

[fritzo]

Thanks @fonnesbeck, that makes sense. No need for an example.

I think we can support deterministic sites via a new .predict_quantiles() method.

class AutoGuide(PyroModule):
    ...
    @torch.no_grad()
    def predict_quantiles(self, quantiles, *args, **kwargs):
        data = self.quantiles(quantiles, *args, **kwargs)
        vectorize = pyro.plate(...)  # as in .quantiles() method
        model = condition(vectorize(self.model), data)
        trace = poutine.trace(model).get_trace(*args, **kwargs)
        return {
            name: site["value"]
            for name, site in trace.nodes.items()
            if site["type"] == "sample"
            if not site_is_subsample(site)
        }

    def predict_median(...):
        # ...similar...

Note there would be a couple difficulties inspecting deterministic sites by default:

1. Deterministic sites require running not only the guide but the model. While it is easy to vectorize the guide, not all models support vectorization. I think the default should make minimal model assumptions.
2. Some models have enormous observation statements. In those cases computing simple guide quantiles of latent variables can be much cheaper than running the model to generate additional deterministic statements. Again I think the default behavior should be the cheap minimal guide-side computation.

For these reasons I think this issue deserves a new separate interface.