Track issues with adjoint

This is to track issues related to using adjoint.

[ ] test_adjoint.py::test_sequential_sum_product_adjoint: xfail_param(MarkovProduct, reason="mysteriously doubles adjoint values?") MarkovProduct is evaluated again at actual.reduce(sum_op) line:

    with AdjointTape() as actual_tape:
        actual = impl(sum_op, prod_op, trans, time, {"prev": "curr"})
        actual = actual.reduce(sum_op)

It might be due to " .reduce binds variables, which triggers alpha-renaming and reinterprets actual?" from slack discussion with @eb8680

[ ] MarkovProduct is not expanded by lazy (or any other Funsor not defined in adjoint_ops). A temporary solution (#492 ) might be to define an adjoint for MarkovProduct. Still work in progress.
[ ] TraceEnum_ELBO uses AdjointTape to calculate expectations. Under eager evaluation nan gradients appear in more than half of the tests as caught by https://github.com/pyro-ppl/pyro/pull/2782:

``` test_enum_funsor.py::test_elbo_plate_plate[2-2] FAILED [ 1%] test_enum_funsor.py::test_elbo_enumerate_1[1] FAILED [ 3%] test_enum_funsor.py::test_elbo_enumerate_1[10] FAILED [ 5%] test_enum_funsor.py::test_elbo_enumerate_2[1] FAILED [ 7%] test_enum_funsor.py::test_elbo_enumerate_2[10] FAILED [ 9%] test_enum_funsor.py::test_elbo_enumerate_3[1] FAILED [ 11%] test_enum_funsor.py::test_elbo_enumerate_3[10] FAILED [ 13%] test_enum_funsor.py::test_elbo_enumerate_plate_1[batch-1] FAILED [ 15%] test_enum_funsor.py::test_elbo_enumerate_plate_1[batch-10] FAILED [ 16%] test_enum_funsor.py::test_elbo_enumerate_plate_1[masked-1] FAILED [ 18%] test_enum_funsor.py::test_elbo_enumerate_plate_1[masked-10] FAILED [ 20%] test_enum_funsor.py::test_elbo_enumerate_plate_2[batch-1] FAILED [ 22%] test_enum_funsor.py::test_elbo_enumerate_plate_2[batch-10] FAILED [ 24%] test_enum_funsor.py::test_elbo_enumerate_plate_2[masked-1] FAILED [ 26%] test_enum_funsor.py::test_elbo_enumerate_plate_2[masked-10] FAILED [ 28%] test_enum_funsor.py::test_elbo_enumerate_plate_3[batch-1] FAILED [ 30%] test_enum_funsor.py::test_elbo_enumerate_plate_3[batch-10] FAILED [ 32%] test_enum_funsor.py::test_elbo_enumerate_plate_3[masked-1] FAILED [ 33%] test_enum_funsor.py::test_elbo_enumerate_plate_3[masked-10] FAILED [ 35%] test_enum_funsor.py::test_elbo_enumerate_plate_4[False-True-1] PASSED [ 37%] test_enum_funsor.py::test_elbo_enumerate_plate_4[False-True-10] PASSED [ 39%] test_enum_funsor.py::test_elbo_enumerate_plate_4[True-False-1] PASSED [ 41%] test_enum_funsor.py::test_elbo_enumerate_plate_4[True-False-10] PASSED [ 43%] test_enum_funsor.py::test_elbo_enumerate_plate_4[True-True-1] PASSED [ 45%] test_enum_funsor.py::test_elbo_enumerate_plate_4[True-True-10] PASSED [ 47%] test_enum_funsor.py::test_elbo_enumerate_plate_5 XFAIL [ 49%] test_enum_funsor.py::test_elbo_enumerate_plate_6[parallel] FAILED [ 50%] test_enum_funsor.py::test_elbo_enumerate_plate_6[sequential] PASSED [ 52%] test_enum_funsor.py::test_elbo_enumerate_plate_7[1] FAILED [ 54%] test_enum_funsor.py::test_elbo_enumerate_plate_7[10] FAILED [ 56%] test_enum_funsor.py::test_elbo_enumerate_plates_1[1] PASSED [ 58%] test_enum_funsor.py::test_elbo_enumerate_plates_1[10] PASSED [ 60%] test_enum_funsor.py::test_elbo_enumerate_plates_2[1] PASSED [ 62%] test_enum_funsor.py::test_elbo_enumerate_plates_2[10] PASSED [ 64%] test_enum_funsor.py::test_elbo_enumerate_plates_3[1] PASSED [ 66%] test_enum_funsor.py::test_elbo_enumerate_plates_3[10] PASSED [ 67%] test_enum_funsor.py::test_elbo_enumerate_plates_4[1] PASSED [ 69%] test_enum_funsor.py::test_elbo_enumerate_plates_4[10] PASSED [ 71%] test_enum_funsor.py::test_elbo_enumerate_plates_5[1] PASSED [ 73%] test_enum_funsor.py::test_elbo_enumerate_plates_5[10] PASSED [ 75%] test_enum_funsor.py::test_elbo_enumerate_plates_6[1] PASSED [ 77%] test_enum_funsor.py::test_elbo_enumerate_plates_6[10] PASSED [ 79%] test_enum_funsor.py::test_elbo_enumerate_plates_7[1] PASSED [ 81%] test_enum_funsor.py::test_elbo_enumerate_plates_7[10] PASSED [ 83%] test_enum_funsor.py::test_elbo_enumerate_plates_8[False-False-1-1] FAILED [ 84%] test_enum_funsor.py::test_elbo_enumerate_plates_8[False-True-1-1] XFAIL [ 86%] test_enum_funsor.py::test_elbo_enumerate_plates_8[True-False-1-1] FAILED [ 88%] test_enum_funsor.py::test_elbo_enumerate_plates_8[True-True-1-1] XFAIL [ 90%] test_enum_funsor.py::test_elbo_enumerate_plate_9 FAILED [ 92%] test_enum_funsor.py::test_elbo_enumerate_plate_10 FAILED [ 94%] test_enum_funsor.py::test_elbo_enumerate_plate_11 FAILED [ 96%] test_enum_funsor.py::test_elbo_enumerate_plate_12 FAILED [ 98%] test_enum_funsor.py::test_elbo_enumerate_plate_13 FAILED [100%] ```

pyro-ppl / funsor

Track issues with adjoint #493