Fix TraceEnum_ELBO jit tests, etc.

[fritzo]

Addresses #1418

• wraps packed.pack() in ignore_jit_warnings
• avoids einsum when not necessary, using tensordot instead
• fixes our einsum patch to allow multiple *operands (high priority fix!)
• tweaks pyro.ops.einsum.paths objective function to not break when opt_einsum releases their next release

Note: although the jit tests are still marked xfail, they should start to pass as soon as we switch to the upcoming opt_einsum 2.3 release.

[eb8680]

Looks like some JIT-related test failures in Dice. I wonder why didn't those appear elsewhere already? Is it from a change in the nightly build?

[fritzo]

Hmm weird, the failures only appear in opt_einsum release; I've been running opt_einsum master locally and tests all pass.

@dgasmith any chance you could cut a minor opt_einsum release soon?

[dgasmith]

Writing the release notes now. Should cut tomorrow.

[dgasmith]

opt_einsum 2.3.1 is now on Conda-forge and PyPI.