[genetic tuning] Do not print confusing stats

Each generation includes two evalution phases (marked with *):
1) Benchmarking of candidates *
2) Breeding
3) Benchmarking of new candidates and selection *

Except for the first generation usually most of the candidates of
step 1) will have been evaluated during step 3) of the previous
generation.

This commit removed the "Jobs(Compiled,GPU) (0,0)/0" that was
printed during step 1) of almost every generation.

In theory it is possible not to have enough valid candidates to
form a new generation. In those cases step 1) will be run and
the progress message will be printed

Author: Theodoros Theodoridis

src/autotuner/genetic_tuning_harness.cc

@@ -539,6 +539,9 @@ void GeneticTunerHarness::runOneGeneration(size_t generation) {
         [](const std::unique_ptr<CandidateConfiguration>& c) {
           return c->runtime != Duration::zero();
         });
+    if (std::distance(firstNew, population.end()) == 0) {
+      return;
+    }
     GeneticSearch::Population newCandidates(
         std::distance(firstNew, population.end()));
     std::move(firstNew, population.end(), newCandidates.begin());