Make pyro.generic.testing usable as a todo list

[fritzo]

Ideally I'd like to implement a very narrow testing interface with two methods that do not mention pytest and do not require testing code to mention pyro.generic. Their use is equivalent to the following:

# in my_pyro_implementation/test/test_generic.py
import pytest
from pyro.generic.testing import get_test_cases, check_test_case
# n.b. do not import any pyro stuff, including pyro.generic

@pytest.mark.parametrize('test_case', get_test_cases())
def test_generic(test_case):
    try:
        check_test_case(test_case)
    except NotImplementedError as e:
        pytest.xfail(f"Not implemented:\n{e}")

I don't care what these are named or whether they are lists or functions or whatever, but I do think we should separate concerns such that:

1. all pyro.generic interface is hidden inside these test helpers, and
2. all testing infra (pytest) is applied outside these test helpers.

I see these helpers as kind of an autoconf mechanism for pyro implementations: running them automatically shows what features are available in a given backend.

(see original discussion in #2049 (comment))

Tasks

• Raise NotImplementedError on missing backend functionality #2055 raise NotImplementedError in pyro.generic if a backend module is missing an attribute
• refactor pyro.generic.testing to free users from mentioning pyro
• add SVI examples to pyro.generic.testing
• exercise these tests with the pyro backend
• exercise these tests with the minipyro backend
• exercise these tests with the numpyro backend
• exercise these tests with the funsor backend

[fritzo]

Moved to pyro-ppl/pyro-api#2