Combine thread launches into single launch per tree for gpu_hist (#4343)

* Combine thread launches into single launch per tree for gpu_hist
algorithm.

* Address deprecation warning

* Add manual column sampler constructor

* Turn off omp dynamic to get a guaranteed number of threads

* Enable openmp in cuda code

include/xgboost/tree_model.h

@@ -62,6 +62,13 @@ struct TreeParam : public dmlc::Parameter<TreeParam> {
     DMLC_DECLARE_FIELD(size_leaf_vector).set_lower_bound(0).set_default(0)
         .describe("Size of leaf vector, reserved for vector tree");
   }
+
+  bool operator==(const TreeParam& b) const {
+    return num_roots == b.num_roots && num_nodes == b.num_nodes &&
+           num_deleted == b.num_deleted && max_depth == b.max_depth &&
+           num_feature == b.num_feature &&
+           size_leaf_vector == b.size_leaf_vector;
+  }
 };
 
 /*! \brief node statistics used in regression tree */
@@ -74,6 +81,10 @@ struct RTreeNodeStat {
   bst_float base_weight;
   /*! \brief number of child that is leaf node known up to now */
   int leaf_child_cnt;
+  bool operator==(const RTreeNodeStat& b) const {
+    return loss_chg == b.loss_chg && sum_hess == b.sum_hess &&
+           base_weight == b.base_weight && leaf_child_cnt == b.leaf_child_cnt;
+  }
 };
 
 /*!
@@ -188,6 +199,11 @@ class RegTree {
       if (is_left_child) pidx |= (1U << 31);
       this->parent_ = pidx;
     }
+    bool operator==(const Node& b) const {
+      return parent_ == b.parent_ && cleft_ == b.cleft_ &&
+             cright_ == b.cright_ && sindex_ == b.sindex_ &&
+             info_.leaf_value == b.info_.leaf_value;
+    }
 
    private:
     /*!
@@ -304,6 +320,11 @@ class RegTree {
     fo->Write(dmlc::BeginPtr(stats_), sizeof(RTreeNodeStat) * nodes_.size());
   }
 
+  bool operator==(const RegTree& b) const {
+    return nodes_ == b.nodes_ && stats_ == b.stats_ &&
+           deleted_nodes_ == b.deleted_nodes_ && param == b.param;
+  }
+
   /**
    * \brief Expands a leaf node into two additional leaf nodes.
    *