Support slicing tree model (#6302)

This PR is meant the end the confusion around best_ntree_limit and unify model slicing. We have multi-class and random forests, asking users to understand how to set ntree_limit is difficult and error prone.

* Implement the save_best option in early stopping.

Co-authored-by: Philip Hyunsu Cho <chohyu01@cs.washington.edu>

doc/python/model.rst

@@ -0,0 +1,38 @@
+#####
+Model
+#####
+
+Slice tree model
+----------------
+
+When ``booster`` is set to ``gbtree`` or ``dart``, XGBoost builds a tree model, which is a
+list of trees and can be sliced into multiple sub-models.
+
+.. code-block:: python
+
+    from sklearn.datasets import make_classification
+    num_classes = 3
+    X, y = make_classification(n_samples=1000, n_informative=5,
+                               n_classes=num_classes)
+    dtrain = xgb.DMatrix(data=X, label=y)
+    num_parallel_tree = 4
+    num_boost_round = 16
+    # total number of built trees is num_parallel_tree * num_classes * num_boost_round
+
+    # We build a boosted random forest for classification here.
+    booster = xgb.train({
+        'num_parallel_tree': 4, 'subsample': 0.5, 'num_class': 3},
+                        num_boost_round=num_boost_round, dtrain=dtrain)
+
+    # This is the sliced model, containing [3, 7) forests
+    # step is also supported with some limitations like negative step is invalid.
+    sliced: xgb.Booster = booster[3:7]
+
+    # Access individual tree layer
+    trees = [_ for _ in booster]
+    assert len(trees) == num_boost_round
+
+
+The sliced model is a copy of selected trees, that means the model itself is immutable
+during slicing.  This feature is the basis of `save_best` option in early stopping
+callback.