Fix expectation computations in TraceEnum_ELBO by using adjoints to calculate marginals

[ordabayevy]

Addresses #2724.
Blocked by pyro-ppl/funsor#413.

This implementation avoids exponentiation of individual terms as suggested by @eb8680 here.

[ordabayevy]

Thanks for reviewing @eb8680 !

[eb8680]

Now that pyro-ppl/funsor#445 has landed can you update the funsor[torch] dependency to the latest master? Should be good to merge if tests pass after that, although performance may be disappointing due to regressions in pyro-ppl/funsor#445.

[ordabayevy]

There is an unrelated to this PR HTTPError: HTTP Error 403: Forbidden from cvae/main.py.

[fritzo]

I'd like to postpone merging this until after Pyro's 1.6 release, so as to avoid the need to release funsor

addressed

Pyro released