[GPU-Plugin] Integration of a faster version of grow_gpu plugin into mainstream (#2360)

* Integrating a faster version of grow_gpu plugin
1. Removed the older files to reduce duplication
2. Moved all of the grow_gpu files under 'exact' folder
3. All of them are inside 'exact' namespace to avoid any conflicts
4. Fixed a bug in benchmark.py while running only 'grow_gpu' plugin
5. Added cub and googletest submodules to ease integration and unit-testing
6. Updates to CMakeLists.txt to directly build cuda objects into libxgboost

* Added support for building gpu plugins through make flow
1. updated makefile and config.mk to add right targets
2. added unit-tests for gpu exact plugin code

* 1. Added support for building gpu plugin using 'make' flow as well
2. Updated instructions for building and testing gpu plugin

* Fix travis-ci errors for PR#2360
1. lint errors on unit-tests
2. removed googletest, instead depended upon dmlc-core provide gtest cache

* Some more fixes to travis-ci lint failures PR#2360

* Added Rory's copyrights to the files containing code from both.

* updated copyright statement as per Rory's request

* moved the static datasets into a script to generate them at runtime

* 1. memory usage print when silent=0
2. tests/ and test/ folder organization
3. removal of the dependency of googletest for just building xgboost
4. coding style updates for .cuh as well

* Fixes for compilation warnings

* add cuda object files as well when JVM_BINDINGS=ON

plugin/updater_gpu/test/cpp/utils.cu

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2017, NVIDIA CORPORATION.  All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#include "utils.cuh"
+
+
+namespace xgboost {
+namespace tree {
+namespace exact {
+
+std::shared_ptr<DMatrix> generateData(const std::string& file) {
+  std::shared_ptr<DMatrix> data(DMatrix::Load(file, false, false, "libsvm"));
+  return data;
+}
+
+std::shared_ptr<DMatrix> preparePluginInputs(const std::string &file,
+                                             std::vector<bst_gpair> *gpair) {
+  std::shared_ptr<DMatrix> dm = generateData(file);
+  gpair->reserve(dm->info().num_row);
+  for (int i = 0; i < dm->info().num_row; ++i) {
+    gpair->push_back(bst_gpair(1.f+static_cast<float>(i%10),
+                               0.5f+static_cast<float>(i%10)));
+  }
+  return dm;
+}
+
+}  // namespace exact
+}  // namespace tree
+}  // namespace xgboost