Configuration for init estimation. (#8343)

* Configuration for init estimation.

* Check whether the model needs configuration based on const attribute `ModelFitted`
instead of a mutable state.
* Add parameter `boost_from_average` to tell whether the user has specified base score.
* Add tests.

src/gbm/gblinear.cc

@@ -95,6 +95,8 @@ class GBLinear : public GradientBooster {
     return model_.num_boosted_rounds;
   }
 
+  bool ModelFitted() const override { return BoostedRounds() != 0; }
+
   void Load(dmlc::Stream* fi) override {
     model_.Load(fi);
   }

src/gbm/gbtree.h

@@ -252,6 +252,10 @@ class GBTree : public GradientBooster {
     return model_.trees.size() / this->LayerTrees();
   }
 
+  bool ModelFitted() const override {
+    return !model_.trees.empty() || !model_.trees_to_update.empty();
+  }
+
   void PredictBatch(DMatrix *p_fmat, PredictionCacheEntry *out_preds,
                     bool training, unsigned layer_begin, unsigned layer_end) override;