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>

plugin/updater_oneapi/README.md

@@ -0,0 +1,42 @@
+# DPC++-based Algorithm for Tree Construction
+This plugin adds support of OneAPI programming model for tree construction and prediction algorithms to XGBoost.
+
+## Usage
+Specify the 'objective' parameter as one of the following options to offload computation of objective function on OneAPI device. 
+
+### Algorithms
+| objective | Description |
+| --- | --- |
+reg:squarederror_oneapi | regression with squared loss  |
+reg:squaredlogerror_oneapi | regression with root mean squared logarithmic loss |
+reg:logistic_oneapi | logistic regression for probability regression task |
+binary:logistic_oneapi | logistic regression for binary classification task |
+binary:logitraw_oneapi | logistic regression for classification, output score before logistic transformation |
+
+Specify the 'predictor' parameter as one of the following options to offload prediction stage on OneAPI device. 
+
+### Algorithms
+| predictor | Description |
+| --- | --- |
+predictor_oneapi | prediction using OneAPI device  |
+
+Please note that parameter names are not finalized and can be changed during further integration of OneAPI support.
+
+Python example:
+```python
+param['predictor'] = 'predictor_oneapi'
+param['objective'] = 'reg:squarederror_oneapi'
+```
+
+## Dependencies
+Building the plugin requires Data Parallel C++ Compiler (https://software.intel.com/content/www/us/en/develop/tools/oneapi/components/dpc-compiler.html)
+
+## Build
+From the command line on Linux starting from the xgboost directory:
+
+```bash
+$ mkdir build
+$ cd build
+$ EXPORT CXX=dpcpp && cmake .. -DPLUGIN_UPDATER_ONEAPI=ON
+$ make -j
+```