Fix contrib.funsor.Trace_EnumELBO model enumeration

[ordabayevy]

Pair coded with @fritzo

Resolves #3046

[fritzo]

🎉

[ordabayevy]

I've found out why the tests used to pass before with an older version of partial_sum_product. The innermost plates would still be eliminated even though they are not in the eliminate. It happens on the second to last line below:

# old partial_sum_product
    while ordinal_to_factors:
        leaf = max(ordinal_to_factors, key=len)
        ...
                if new_plates == leaf:
                    raise ValueError("intractable!")
                f = f.reduce(prod_op, leaf - new_plates)  # innermost "data" plate in test_elbo_enumerate_plate_1 eliminated here
                ordinal_to_factors[new_plates].append(f)

which is probably less intuitive then the current version of partial_sum_product where all eliminated plates have to be passed explicitly in eliminate.