Enable loading model from <1.0.0 trained with objective='binary:logitraw' (#6517) (#6524)

* Enable loading model from <1.0.0 trained with objective='binary:logitraw'

* Add binary:logitraw in model compatibility testing suite

* Feedback from @trivialfis: Override ProbToMargin() for LogisticRaw

Co-authored-by: Jiaming Yuan <jm.yuan@outlook.com>

Co-authored-by: Philip Hyunsu Cho <chohyu01@cs.washington.edu>

src/objective/regression_loss.h

@@ -162,6 +162,9 @@ struct LogisticRaw : public LogisticRegression {
     predt = common::Sigmoid(predt);
     return std::max(predt * (T(1.0f) - predt), eps);
   }
+  static bst_float ProbToMargin(bst_float base_score) {
+    return base_score;
+  }
   static const char* DefaultEvalMetric() { return "auc"; }
 
   static const char* Name() { return "binary:logitraw"; }