[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/python-gpu/test_gpu_ranking.py

@@ -29,7 +29,6 @@ def comp_training_with_rank_objective(
         "booster": "gbtree",
         "tree_method": "gpu_hist",
         "gpu_id": 0,
-        "predictor": "gpu_predictor",
     }
 
     num_trees = 100
@@ -54,7 +53,6 @@ def comp_training_with_rank_objective(
         "booster": "gbtree",
         "tree_method": "hist",
         "gpu_id": -1,
-        "predictor": "cpu_predictor",
     }
     cpu_params["objective"] = rank_objective
     cpu_params["eval_metric"] = metric_name