Fix training continuation with categorical model. (#7810)

* Make sure the task is initialized before construction of tree updater.

This is a quick fix meant to be backported to 1.6, for a full fix we should pass the model
param into tree updater by reference instead.

src/learner.cc

@@ -419,6 +419,7 @@ class LearnerConfiguration : public Learner {
       obj_.reset(ObjFunction::Create(tparam_.objective, &generic_parameters_));
     }
     obj_->LoadConfig(objective_fn);
+    learner_model_param_.task = obj_->Task();
 
     tparam_.booster = get<String>(gradient_booster["name"]);
     if (!gbm_) {