[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/load_pickle.py

@@ -1,5 +1,5 @@
-'''Loading a pickled model generated by test_pickling.py, only used by
-`test_gpu_with_dask.py`'''
+"""Loading a pickled model generated by test_pickling.py, only used by
+`test_gpu_with_dask.py`"""
 import json
 import os
 
@@ -12,9 +12,9 @@ from xgboost import testing as tm
 
 
 class TestLoadPickle:
-    def test_load_pkl(self):
-        '''Test whether prediction is correct.'''
-        assert os.environ['CUDA_VISIBLE_DEVICES'] == '-1'
+    def test_load_pkl(self) -> None:
+        """Test whether prediction is correct."""
+        assert os.environ["CUDA_VISIBLE_DEVICES"] == "-1"
         bst = load_pickle(model_path)
         x, y = build_dataset()
         if isinstance(bst, xgb.Booster):
@@ -28,46 +28,42 @@ class TestLoadPickle:
 
         assert len(res) == 10
 
-    def test_predictor_type_is_auto(self):
-        '''Under invalid CUDA_VISIBLE_DEVICES, predictor should be set to
-        auto'''
-        assert os.environ['CUDA_VISIBLE_DEVICES'] == '-1'
+    def test_context_is_removed(self) -> None:
+        """Under invalid CUDA_VISIBLE_DEVICES, context should reset"""
+        assert os.environ["CUDA_VISIBLE_DEVICES"] == "-1"
         bst = load_pickle(model_path)
         config = bst.save_config()
         config = json.loads(config)
-        assert config['learner']['gradient_booster']['gbtree_train_param'][
-            'predictor'] == 'auto'
+        assert config["learner"]["generic_param"]["gpu_id"] == "-1"
 
-    def test_predictor_type_is_gpu(self):
-        '''When CUDA_VISIBLE_DEVICES is not specified, keep using
-        `gpu_predictor`'''
-        assert 'CUDA_VISIBLE_DEVICES' not in os.environ.keys()
+    def test_context_is_preserved(self) -> None:
+        """Test the device context is preserved after pickling."""
+        assert "CUDA_VISIBLE_DEVICES" not in os.environ.keys()
         bst = load_pickle(model_path)
         config = bst.save_config()
         config = json.loads(config)
-        assert config['learner']['gradient_booster']['gbtree_train_param'][
-            'predictor'] == 'gpu_predictor'
+        assert config["learner"]["generic_param"]["gpu_id"] == "0"
 
-    def test_wrap_gpu_id(self):
-        assert os.environ['CUDA_VISIBLE_DEVICES'] == '0'
+    def test_wrap_gpu_id(self) -> None:
+        assert os.environ["CUDA_VISIBLE_DEVICES"] == "0"
         bst = load_pickle(model_path)
         config = bst.save_config()
         config = json.loads(config)
-        assert config['learner']['generic_param']['gpu_id'] == '0'
+        assert config["learner"]["generic_param"]["gpu_id"] == "0"
 
         x, y = build_dataset()
         test_x = xgb.DMatrix(x)
         res = bst.predict(test_x)
         assert len(res) == 10
 
-    def test_training_on_cpu_only_env(self):
-        assert os.environ['CUDA_VISIBLE_DEVICES'] == '-1'
+    def test_training_on_cpu_only_env(self) -> None:
+        assert os.environ["CUDA_VISIBLE_DEVICES"] == "-1"
         rng = np.random.RandomState(1994)
         X = rng.randn(10, 10)
         y = rng.randn(10)
         with tm.captured_output() as (out, err):
             # Test no thrust exception is thrown
             with pytest.raises(xgb.core.XGBoostError):
-                xgb.train({'tree_method': 'gpu_hist'}, xgb.DMatrix(X, y))
+                xgb.train({"tree_method": "gpu_hist"}, xgb.DMatrix(X, y))
 
-            assert out.getvalue().find('No visible GPU is found') != -1
+            assert out.getvalue().find("No visible GPU is found") != -1