GitLab

* basic gpu_linear_tree_learner implementation

* corresponding config of gpu linear tree

* Update src/io/config.cpp
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

* work around for gpu linear tree learner without gpu enabled

* add #endif

* add #ifdef USE_GPU

* fix lint problems

* fix compilation when USE_GPU is OFF

* add destructor

* add gpu_linear_tree_learner.cpp in make file list

* use template for linear tree learner

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Co-authored-by: Nikita Titov <nekit94-08@mail.ru>
Co-authored-by: shiyu1994 <shiyu_k1994@qq.com>