Porting elementwise metrics to GPU. (#3952)

* Port elementwise metrics to GPU.

* All elementwise metrics are converted to static polymorphic.
* Create a reducer for metrics reduction.
* Remove const of Metric::Eval to accommodate CubMemory.

src/common/math.h

@@ -127,7 +127,7 @@ inline bool CheckNAN(T v) {
 #endif
 }
 template<typename T>
-inline T LogGamma(T v) {
+XGBOOST_DEVICE inline T LogGamma(T v) {
 #ifdef _MSC_VER
 #if _MSC_VER >= 1800
   return lgamma(v);