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/test_distributed/test_with_dask/test_with_dask.py

@@ -1168,7 +1168,7 @@ def test_dask_aft_survival() -> None:
 
 def test_dask_ranking(client: "Client") -> None:
     dpath = "demo/rank/"
-    mq2008 = tm.get_mq2008(dpath)
+    mq2008 = tm.data.get_mq2008(dpath)
     data = []
     for d in mq2008:
         if isinstance(d, scipy.sparse.csr_matrix):