Fix model slicing. (#7149)

* Use correct pointer.
* Remove best_iteration/best_score.

tests/python/test_cli.py

@@ -99,6 +99,8 @@ eval[test] = {data_path}
             # CLI model doesn't contain feature info.
             booster.feature_names = None
             booster.feature_types = None
+            booster.set_attr(best_iteration=None)
+            booster.set_attr(best_ntree_limit=None)
 
             booster.save_model(model_out_py)
             py_predt = booster.predict(data)

tests/python/test_data_iterator.py

@@ -114,7 +114,8 @@ def run_data_iterator(
     if tree_method != "gpu_hist":
         rtol = 1e-1  # flaky
     else:
-        np.testing.assert_allclose(it_predt, arr_predt, rtol=1e-3)
+        # Model can be sensitive to quantiles, use 1e-2 to relax the test.
+        np.testing.assert_allclose(it_predt, arr_predt, rtol=1e-2)
         rtol = 1e-6
 
     np.testing.assert_allclose(