[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/common/test_json.cc

@@ -41,7 +41,6 @@ std::string GetModelStr() {
     "num_class": "0",
     "num_feature": "10",
     "objective": "reg:linear",
-    "predictor": "gpu_predictor",
     "tree_method": "gpu_hist",
     "updater": "grow_gpu_hist"
   },