This PR refactors the three base scorers (_PredictScorer, _ProbaScorer, _ThresholdScorer) into _BaseScorer. These are all private classes so we can remove them directly.
Honestly I'm not sure whether we want this (vote +0 for myself :))

• Advantages:

  • avoid duplicate code, easier to maintain
  • avoid inconsistency between different base scorers (e.g., previously, for binary y_true, _ProbaScorer requires score function to accept 2d y_pred, but _ThresholdScorer requires score function to accept 1d y_pred, see DOC make_scorer now requires score function to accpet 1d y_pred when needs_proba is True #14318)

• Disadvantages:

  • Maybe more difficult if we want to add more base scorers?