[breaking] Remove the predictor param, allow fallback to prediction using DMatrix. (#9129)

- A `DeviceOrd` struct is implemented to indicate the device. It will eventually replace the `gpu_id` parameter.
- The `predictor` parameter is removed.
- Fallback to `DMatrix` when `inplace_predict` is not available.
- The heuristic for choosing a predictor is only used during training.

tests/cpp/test_learner.cc

@@ -342,16 +342,6 @@ TEST(Learner, GPUConfiguration) {
     learner->UpdateOneIter(0, p_dmat);
     ASSERT_EQ(learner->Ctx()->gpu_id, 0);
   }
-  {
-    // With CPU algorithm but GPU Predictor, this is to simulate when
-    // XGBoost is only used for prediction, so tree method is not
-    // specified.
-    std::unique_ptr<Learner> learner {Learner::Create(mat)};
-    learner->SetParams({Arg{"tree_method", "hist"},
-                        Arg{"predictor", "gpu_predictor"}});
-    learner->UpdateOneIter(0, p_dmat);
-    ASSERT_EQ(learner->Ctx()->gpu_id, 0);
-  }
 }
 #endif  // defined(XGBOOST_USE_CUDA)