Support categorical data for hist. (#7695)

* Extract partitioner from hist.
* Implement categorical data support by passing the gradient index directly into the partitioner.
* Organize/update document.
* Remove code for negative hessian.

tests/cpp/tree/test_partitioner.h

@@ -0,0 +1,21 @@
+/*!
+ * Copyright 2021-2022, XGBoost contributors.
+ */
+#include <xgboost/tree_model.h>
+#include <vector>
+#include "../../../src/tree/hist/expand_entry.h"
+
+namespace xgboost {
+namespace tree {
+inline void GetSplit(RegTree *tree, float split_value, std::vector<CPUExpandEntry> *candidates) {
+  tree->ExpandNode(
+      /*nid=*/RegTree::kRoot, /*split_index=*/0, /*split_value=*/split_value,
+      /*default_left=*/true, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f,
+      /*left_sum=*/0.0f,
+      /*right_sum=*/0.0f);
+  candidates->front().split.split_value = split_value;
+  candidates->front().split.sindex = 0;
+  candidates->front().split.sindex |= (1U << 31);
+}
+}  // namespace tree
+}  // namespace xgboost