Pass obj info by reference instead of by value. (#8889)

- Pass obj info into tree updater as const pointer.

This way we don't have to initialize the learner model param before configuring gbm, hence
breaking up the dependency of configurations.

src/tree/updater_sync.cc

@@ -1,5 +1,5 @@
 /**
- * Copyright 2014-2013 by XBGoost Contributors
+ * Copyright 2014-2023 by XBGoost Contributors
  * \file updater_sync.cc
  * \brief synchronize the tree in all distributed nodes
  */
@@ -53,5 +53,5 @@ class TreeSyncher : public TreeUpdater {
 
 XGBOOST_REGISTER_TREE_UPDATER(TreeSyncher, "sync")
     .describe("Syncher that synchronize the tree in all distributed nodes.")
-    .set_body([](Context const* ctx, ObjInfo) { return new TreeSyncher(ctx); });
+    .set_body([](Context const* ctx, auto) { return new TreeSyncher(ctx); });
 }  // namespace xgboost::tree