Change model prediction output from tuple to dict

[streino]

Change originally proposed in #73

Current model outputs a (scores, labels) prediction tuple.

I suggest changing it to a {'labels': labels, 'scores': scores} dict. This makes output more explicit and follows what we've done training-side in the dataset format and model inputs.

It will change the model output in serving as well, so it would require some matching changes in the code that uses the model for prediction.