Initial support for multi-target tree. (#8616)

* Implement multi-target for hist.

- Add new hist tree builder.
- Move data fetchers for tests.
- Dispatch function calls in gbm base on the tree type.

tests/python-gpu/test_gpu_ranking.py

@@ -116,7 +116,7 @@ def test_with_mq2008(objective, metric) -> None:
         x_valid,
         y_valid,
         qid_valid,
-    ) = tm.get_mq2008(os.path.join(os.path.join(tm.demo_dir(__file__), "rank")))
+    ) = tm.data.get_mq2008(os.path.join(os.path.join(tm.demo_dir(__file__), "rank")))
 
     if metric.find("map") != -1 or objective.find("map") != -1:
         y_train[y_train <= 1] = 0.0