Use a user's existing ratings as predictions. This rating predictor
returns the user's rating as a prediction for all items the user has rated,
and returns no prediction for items the user has rated. It is useful in
conjunction with a FallbackItemScorer to return a user's rating
when it is known.
Predict for items in a vector. The key domain of the provided vector is the items whose
predictions are requested, and the predict method sets the values for each item to its
predict (or unsets it, if no prediction can be provided). The previous values are discarded.
user - The user ID.
predictions - The prediction output vector. Its key domain is the items to score.