[GPU-Plugin] Various fixes (#2579)

* Fix test large

* Add check for max_depth 0

* Update readme

* Add LBS specialisation for dense data

* Add bst_gpair_precise

* Temporarily disable accuracy tests on test_large.py

* Solve unused variable compiler warning

* Fix max_bin > 1024 error

src/gbm/gbtree_model.h

@@ -117,7 +117,6 @@ struct GBTreeModel {
   }
   void CommitModel(std::vector<std::unique_ptr<RegTree> >&& new_trees,
                    int bst_group) {
-    size_t old_ntree = trees.size();
     for (size_t i = 0; i < new_trees.size(); ++i) {
       trees.push_back(std::move(new_trees[i]));
       tree_info.push_back(bst_group);