Use correct event_dim in masked Pyro sample statements

[eb8680]

In https://github.com/pyro-ppl/pyro/blob/dev/pyro/primitives.py#L102 the batch shape of the input distribution is ignored when returning a final interleaved deterministic value, which may cause issues with MultiWorldCounterfactual's use and abuse of plates. The following change should be sufficient to prevent any issues:

-    return deterministic(name, value)
+    return deterministic(name, value, event_dim=len(fn.event_shape))

[eb8680]

I believe this is unnecessary following the changes to MultiWorldCounterfactual in #48, which makes the workaround of pyro.poutine.blocking the deterministic site introduced when handling obs_mask the more correct behavior.

[eb8680]

Obsolete following #96 and #97 which remove use of obs_mask