Enable distributed GPU training over Rabit (#7930)

src/common/common.h

@@ -274,6 +274,24 @@ template <typename Indexable>
 XGBOOST_DEVICE size_t LastOf(size_t group, Indexable const &indptr) {
   return indptr[group + 1] - 1;
 }
+
+/**
+ * @brief A CRTP (curiously recurring template pattern) helper function.
+ *
+ * https://www.fluentcpp.com/2017/05/19/crtp-helper/
+ *
+ * Does two things:
+ * 1. Makes "crtp" explicit in the inheritance structure of a CRTP base class.
+ * 2. Avoids having to `static_cast` in a lot of places.
+ *
+ * @tparam T The derived class in a CRTP hierarchy.
+ */
+template <typename T>
+struct Crtp {
+  T &Underlying() { return static_cast<T &>(*this); }
+  T const &Underlying() const { return static_cast<T const &>(*this); }
+};
+
 }  // namespace common
 }  // namespace xgboost
 #endif  // XGBOOST_COMMON_COMMON_H_