Support plates in RenyiELBO

[fritzo]

Addresses #2220
Pair coded with @martinjankowiak

This adds plate support to RenyiELBO. Once this merges, similar changes can be applied to TraceAdaptiveTail_ELBO and ReweightedWakeSleep.

Tested

• strengthened inference tests
• added weak tests with various batch shapes
• ... other test suggestions welcome...

[fritzo]

cc @tuananhle7 @iffsid

[iffsid]

cool, thanks!
We'll setup a PR once this merges. :)

[fehiepsi]

The change looks reasonable to me. Just to verify my understanding, to support the previous behavior, I should declare different plates for z and x?

def model():
    with pyro.plate('z', 2):
        pyro.sample('z', ...)
    with pyro.plate('x', 2):
        pyro.sample('x', ...)

[fritzo]

@fehiepsi yes good point, you can indeed declare different plates to achieve previous behavior.