Fix _check_start_shape and BinaryMetropolis.astep

[ricardoV94]

This PR fixes two small issues in the methods mentioned in the PR title.

Two tests are now failing because they end in a call to az.summary, is this known to still be broken?

[ricardoV94]

Do we need to pass the value_var explicitly, or it works as well with the rv_var (i.e., model["z"])?

The test passes but I didn't have the patience to check if it affected sampling.

The ArrayStep docs are ambiguous in this regard:

    Blocked step method that is generalized to accept vectors of variables.

    Parameters
    ----------
    vars: list
        List of variables for sampler.

[brandonwillard]

The value variables are given to the step methods. See here.

[ricardoV94]

I meant when specifying manually the steps as in this test

[brandonwillard]

In general, step methods require value variables as their vars argument/field, so it looks like your manual setup for that step method in the test is correct.

[ricardoV94]

Thanks, that was exactly my hunch.

Should we add some logic where we check if a variable manually assigned to a stepper contains a value_var and if so use that instead?

That would make manual assignments less cumbersome and error prone. What do you think?

[brandonwillard]

We only need to state what kind of variables they should be in a docstring. Aside from that, we could perhaps check that the vars are actually value variables in the step method's model.

[twiecki]

Thanks @ricardoV94!