Disallow multiple roots for tree_method=hist (dmlc#1979)

As discussed in issue dmlc#1978, tree_method=hist ignores the parameter
param.num_roots; it simply assumes that the tree has only one root. In
particular, when InitData() method initializes row_set_collection_, it simply
assigns all rows to node 0, the value that's hard-coded.

For now, the updater will simply fail when num_roots exceeds 1. I will revise
the updater soon to support multiple roots.

src/tree/tree_updater.cc

@@ -29,6 +29,7 @@ DMLC_REGISTRY_LINK_TAG(updater_colmaker);
 DMLC_REGISTRY_LINK_TAG(updater_skmaker);
 DMLC_REGISTRY_LINK_TAG(updater_refresh);
 DMLC_REGISTRY_LINK_TAG(updater_prune);
+DMLC_REGISTRY_LINK_TAG(updater_fast_hist);
 DMLC_REGISTRY_LINK_TAG(updater_histmaker);
 DMLC_REGISTRY_LINK_TAG(updater_sync);
 }  // namespace tree

src/tree/updater_fast_hist.cc

@@ -139,6 +139,9 @@ class FastHistMaker: public TreeUpdater {
       tstart = dmlc::GetTime();
       this->InitData(gmat, gpair, *p_fmat, *p_tree);
       time_init_data = dmlc::GetTime() - tstart;
+      // FIXME(hcho3): this code is broken when param.num_roots > 1. Please fix it
+      CHECK_EQ(p_tree->param.num_roots, 1)
+        << "tree_method=hist does not support multiple roots at this moment";
       for (int nid = 0; nid < p_tree->param.num_roots; ++nid) {
         tstart = dmlc::GetTime();
         hist_.AddHistRow(nid);