[TMVA] BDTG -- Parallelise application get values

Simple to //-ise over number of output classes. Doing the same over
number of trees seems to yield no benefit for ~1000 depth=3 tree so this
was skipped (binary and multi class).

Optimal would be to //-ise over events, but the current design makes
this tricky.

tmva/tmva/src/MethodBDT.cxx

@@ -2502,6 +2502,18 @@ const std::vector<Float_t>& TMVA::MethodBDT::GetMulticlassValues()
    UInt_t nClasses = DataInfo().GetNClasses();
    std::vector<Double_t> temp(nClasses);
    auto forestSize = fForest.size();
+
+   #ifdef R__USE_IMT
+   std::vector<TMVA::DecisionTree *> forest = fForest;
+   auto get_output = [&e, &forest, &temp, forestSize, nClasses](UInt_t iClass) {
+      for (UInt_t itree = iClass; itree < forestSize; itree += nClasses) {
+         temp[iClass] += forest[itree]->CheckEvent(e, kFALSE);
+      }
+   };
+
+   TMVA::Config::Instance().GetThreadExecutor()
+                           .Foreach(get_output, ROOT::TSeqU(nClasses));
+   #else
    // trees 0, nClasses, 2*nClasses, ... belong to class 0
    // trees 1, nClasses+1, 2*nClasses+1, ... belong to class 1 and so forth
    UInt_t classOfTree = 0;
@@ -2509,6 +2521,7 @@ const std::vector<Float_t>& TMVA::MethodBDT::GetMulticlassValues()
       temp[classOfTree] += fForest[itree]->CheckEvent(e, kFALSE);
       if (++classOfTree == nClasses) classOfTree = 0; // cheap modulo
    }
+   #endif
 
    // we want to calculate sum of exp(temp[j] - temp[i]) for all i,j (i!=j)
    // first calculate exp(), then replace minus with division.