[dask] Add DaskXGBRanker (#6576)

* Initial support for distributed LTR using dask.

* Support `qid` in libxgboost.
* Refactor `predict` and `n_features_in_`, `best_[score/iteration/ntree_limit]`
  to avoid duplicated code.
* Define `DaskXGBRanker`.

The dask ranker doesn't support group structure, instead it uses query id and
convert to group ptr internally.

tests/python/test_dmatrix.py

@@ -239,6 +239,19 @@ class TestDMatrix:
         dtrain.get_float_info('base_margin')
         dtrain.get_uint_info('group_ptr')
 
+    def test_qid(self):
+        rows = 100
+        cols = 10
+        X, y = rng.randn(rows, cols), rng.randn(rows)
+        qid = rng.randint(low=0, high=10, size=rows, dtype=np.uint32)
+        qid = np.sort(qid)
+
+        Xy = xgb.DMatrix(X, y)
+        Xy.set_info(qid=qid)
+        group_ptr = Xy.get_uint_info('group_ptr')
+        assert group_ptr[0] == 0
+        assert group_ptr[-1] == rows
+
     def test_feature_weights(self):
         kRows = 10
         kCols = 50