Fix TraceEnum_ELBO ignoring is_auxiliary option

[juanitorduz]

Closes #2013

Summary

TraceEnum_ELBO crashed with a KeyError when the guide contained sample sites marked with infer={"is_auxiliary": True} that had no corresponding site in the model. This is a common pattern for constructing flexible variational families (e.g., sampling from a multivariate normal auxiliary and feeding it through Delta distributions).

Root Cause

In get_importance_trace_enum (numpyro/infer/elbo.py), when processing guide sites, the code unconditionally accessed model_trace[name] without first checking whether the site exists in the model trace:

elif not is_model and (model_trace[name].get("kl") is not None):

[fehiepsi]

Thanks for the fix!