public void addInput(Reference reference, Input input) {
        if (inputs.containsKey(reference)) {
            Input existing = inputs().get(reference);
            if (! input.equals(existing))
                throw new IllegalArgumentException("Duplicate input: Has both " + input + " and existing " + existing);
        }
        inputs.put(reference, input);
    }
Adds the type of an input feature consumed by this profile.
All inputs must either be declared through this or in query profile types,
otherwise they are assumes to be scalars.
config-model/src/main/java/com/yahoo/schema/RankProfile.java