Failures in combining preempt with enumeration-based inference in Pyro

[eb8680]

In the course of working on #240 and #236, @rfl-urbaniak ran into problems when attempting to use infer_discrete and TraceEnum_ELBO in conjunction with causation and explanation queries defined via chirho.counterfactual.ops.preempt. This issue tracks progress on diagnosing and fixing those problems.

[eb8680]

@rfl-urbaniak it's been a few weeks since you showed us this, and I'm afraid I've lost track of the context since then. Could you please create a failing test that illustrates this behavior and paste it here or push it to a branch in this repository and put a link to the branch here? Once you do that I will work on a fix.

[rfl-urbaniak]

A pytest test capturing the problem is now available in #253, also listed in the main description of that PR.

[rfl-urbaniak]

@eb8680 A most recent version of a notebook illustrating what seems to be a problem with using enumeration with explanation-related handlers is now in the ac-archive (PR #403 ), in the enumeration-explanation.ipynb notebook. If this gets resolved, the notebooks will need some updating, otherwise, I'm using sampling at least for now, and this shouldn't make much difference, as the number of possible combinations in the examples used in the notebooks is somewhat limited.