It is not possible since the score function returns `some_metric(self.predict(X), y)` and there is no way to extract the `some_metric` function. It has bothered me more than once and I feel that it would be nice to decouple the `self.score` method into a `self.score` method + a `self.score_metric` method that would be called by `self.loss`. That way, one could directly call `some_metric` in situations like this. (This pattern also occurs when computing the oob scores in the forest module.)