Decide how to deal with automatic broadcast in derived RVs

[ricardoV94]

Both #26 and #22 raise the question of how to deal with automatic RVs whose value_variable results from an intermediate broadcast of the base RV

loc = at.vector("loc")
y_base_rv = at.random.normal(0, 1, name="y_base_rv", size=2)
y_rv = loc + y_base_rv
y_val = y_rv.clone()

logp = joint_logprob(y_rv, {y_rv: y_val})
logp.eval({y_val: [0, 0, 0, 0], loc_val: [0, 0, 0, 0]})

The computed logp is only correct when loc is of size==2.
This might be even more complicated if the y_base_rv inputs are themselves variable (meaning it may itself have not a fixed size)

I see two ways we could deal with this:

1. Add a shape assert for y_base_rv.shape == y_val.shape in the returned logprob
2. Ignore the situation and put the onus on the users to specify correctly sized variables for derived logprobs
3. Something else?

[brandonwillard]

We defer to Aesara on these matters.

In other words, we let Aesara's error handling explain the issue to the user. It has special error messages for exactly these kinds of user-induced issues, so there's no reason to introduce redundancies to that effect in this library.