Implementation needed for computing a custom target quantity

[florence-bockting]

In the function for specifying your target quantities el.target, the possibility to specify a custom target quantity using the target_method argument is not yet implemented:

el.target(
            name="custom_target",
            target_method=custom_func(user_arg, **kwargs), # this function hasn't been implemented yet
            query=el.queries.quantile((25, 50, 75)),
            loss=el.losses.MMD2(kernel="energy"),
            weight=1.0
        )

Note that we need to differentiate between function arguments that are set by the user and those that come from previous workflow steps (e.g. output of the generative model).