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.

tests/cpp/metric/test_elementwise_metric.cu

@@ -0,0 +1,5 @@
+/*!
+ * Copyright 2018 XGBoost contributors
+ */
+// Dummy file to keep the CUDA conditional compile trick.
+#include "test_elementwise_metric.cc"