Remove public access to tree model param. (#8902)

* Make tree model param a private member.
* Number of features and targets are immutable after construction.

This is to reduce the number of places where we can run configuration.

src/predictor/cpu_predictor.cc

@@ -275,7 +275,7 @@ float FillNodeMeanValues(RegTree const *tree, bst_node_t nidx, std::vector<float
 }
 
 void FillNodeMeanValues(RegTree const* tree, std::vector<float>* mean_values) {
-  size_t num_nodes = tree->param.num_nodes;
+  size_t num_nodes = tree->NumNodes();
   if (mean_values->size() == num_nodes) {
     return;
   }