Allow non-vectorized function arguments in Batch

Do I understand correctly that`BatchOp` assumes all function arguments should be vectorized?

If so, can we allow arguments that aren't, a bit like the inactive args in grad (I think)? I mainly want it because I'm passing values to my function that were intended to be captured by the function, but since this isn't possible, I pass them as arguments. Not having to broadcast them first is convenient and intuitive. More importantly for most people, perhaps, is that it's more efficient, because we don't broadcast values prematurely. I may want to do long calculations on the values that aren't vectorized _before_ they interact with vectorized values, and doing those calculations vectorized is a waste.

Start by reading the BatchOp behavior and comparing it with the inactive arguments in grad. Determine how non-vectorized arguments should be represented and propagated without premature broadcasting, then add coverage showing that long calculations on those arguments remain non-vectorized before interacting with vectorized values.

Written by the indexing model from the issue text.