Introducing DPC++-based plugin (predictor, objective function) supporting oneAPI programming model (#5825)

* Added plugin with DPC++-based predictor and objective function

* Update CMakeLists.txt

* Update regression_obj_oneapi.cc

* Added README.md for OneAPI plugin

* Added OneAPI predictor support to gbtree

* Update README.md

* Merged kernels in gradient computation. Enabled multiple loss functions with DPC++ backend

* Aligned plugin CMake files with latest master changes. Fixed whitespace typos

* Removed debug output

* [CI] Make oneapi_plugin a CMake target

* Added tests for OneAPI plugin for predictor and obj. functions

* Temporarily switched to default selector for device dispacthing in OneAPI plugin to enable execution in environments without gpus

* Updated readme file.

* Fixed USM usage in predictor

* Removed workaround with explicit templated names for DPC++ kernels

* Fixed warnings in plugin tests

* Fix CMake build of gtest

Co-authored-by: Hyunsu Cho <chohyu01@cs.washington.edu>

src/common/common.h

@@ -154,6 +154,12 @@ inline void AssertGPUSupport() {
 #endif  // XGBOOST_USE_CUDA
 }
 
+inline void AssertOneAPISupport() {
+#ifndef XGBOOST_USE_ONEAPI
+    LOG(FATAL) << "XGBoost version not compiled with OneAPI support.";
+#endif  // XGBOOST_USE_ONEAPI
+}
+
 }  // namespace common
 }  // namespace xgboost
 #endif  // XGBOOST_COMMON_COMMON_H_