Make golds, preds, probs optional in Scorer.score()

[vincentschen]

Is your feature request related to a problem? Please describe.

Passing golds, preds, and probs are all required to use Scorer.score.
Often, preds and probs are not both available, and requires additional usage of a util function — AKA not the best interface!

Describe the solution you'd like

Make certain fields (preds / probs) optional in Scorer.score, depending on usage.

[bhancock8]

This should actually be a pretty easy fix. Scorer.score() calls metric_score() which can be passed any combination of golds, preds, and probs and it'll complain with an error if the metric you requested didn't get all of the inputs it needs to compute. So I think all we need to do here is update the signature to let all three of those be optional.