Implement categorical prediction for CPU and GPU predict leaf. (#7001)

* Categorical prediction with CPU predictor and GPU predict leaf.

* Implement categorical prediction for CPU prediction.
* Implement categorical prediction for GPU predict leaf.
* Refactor the prediction functions to have a unified get next node function.

Co-authored-by: Shvets Kirill <kirill.shvets@intel.com>

src/predictor/predict_fn.h

@@ -0,0 +1,31 @@
+/*!
+ * Copyright 2021 by XGBoost Contributors
+ */
+#ifndef XGBOOST_PREDICTOR_PREDICT_FN_H_
+#define XGBOOST_PREDICTOR_PREDICT_FN_H_
+#include "../common/categorical.h"
+#include "xgboost/tree_model.h"
+
+namespace xgboost {
+namespace predictor {
+template <bool has_missing, bool has_categorical>
+inline XGBOOST_DEVICE bst_node_t
+GetNextNode(const RegTree::Node &node, const bst_node_t nid, float fvalue,
+            bool is_missing, RegTree::CategoricalSplitMatrix const &cats) {
+  if (has_missing && is_missing) {
+    return node.DefaultChild();
+  } else {
+    if (has_categorical && common::IsCat(cats.split_type, nid)) {
+      auto node_categories = cats.categories.subspan(cats.node_ptr[nid].beg,
+                                                     cats.node_ptr[nid].size);
+      return Decision(node_categories, common::AsCat(fvalue))
+                 ? node.LeftChild()
+                 : node.RightChild();
+    } else {
+      return node.LeftChild() + !(fvalue < node.SplitCond());
+    }
+  }
+}
+}      // namespace predictor
+}      // namespace xgboost
+#endif  // XGBOOST_PREDICTOR_PREDICT_FN_H_