make xvalidation always leave state of machine/features invariant

previously, this was only done for number of threads >1 which lead to inconsistent behaviour

src/shogun/evaluation/CrossValidation.cpp

@@ -241,24 +241,14 @@ float64_t CCrossValidation::evaluate_one_run(
 			CrossValidationFoldStorage* fold = new CrossValidationFoldStorage();
 			SG_REF(fold)
 
-			CMachine* machine;
-			CFeatures* features;
-			CLabels* labels;
-			CEvaluation* evaluation_criterion;
+			auto machine = (CMachine*)m_machine->clone();
 
-			if (get_global_parallel()->get_num_threads() == 1)
-			{
-				machine = m_machine;
-				features = m_features;
-				evaluation_criterion = m_evaluation_criterion;
-			}
-			else
-			{
-				machine = (CMachine*)m_machine->clone();
-				features = (CFeatures*)m_features->clone();
-				evaluation_criterion =
-				    (CEvaluation*)m_evaluation_criterion->clone();
-			}
+			// TODO while these are not used through const interfaces,
+			// we unfortunately have to clone, even though these could be shared
+			auto features = (CFeatures*)m_features->clone();
+			auto labels = (CLabels*)m_labels->clone();
+			auto evaluation_criterion =
+			    (CEvaluation*)m_evaluation_criterion->clone();
 
 			/* evtl. update xvalidation output class */
 			fold->set_run_index(index);
@@ -271,10 +261,6 @@ float64_t CCrossValidation::evaluate_one_run(
 			features->add_subset(inverse_subset_indices);
 
 			/* set label subset for training */
-			if (get_global_parallel()->get_num_threads() == 1)
-				labels = m_labels;
-			else
-				labels = machine->get_labels();
 			labels->add_subset(inverse_subset_indices);
 
 			SG_DEBUG("training set %d:\n", i)
@@ -287,6 +273,7 @@ float64_t CCrossValidation::evaluate_one_run(
 
 			/* train machine on training features and remove subset */
 			SG_DEBUG("starting training\n")
+			machine->set_labels(labels);
 			machine->train(features);
 			SG_DEBUG("finished training\n")
 
@@ -340,13 +327,10 @@ float64_t CCrossValidation::evaluate_one_run(
 
 			/* clean up, remove subsets */
 			labels->remove_subset();
-			if (get_global_parallel()->get_num_threads() != 1)
-			{
-				SG_UNREF(machine);
-				SG_UNREF(features);
-				SG_UNREF(labels);
-				SG_UNREF(evaluation_criterion);
-			}
+			SG_UNREF(machine);
+			SG_UNREF(features);
+			SG_UNREF(labels);
+			SG_UNREF(evaluation_criterion);
 			SG_UNREF(result_labels);
 			SG_UNREF(fold)
 		}