message: Log the poplar engine's compilation progress.
  Test Plan: It compiles and shows the poplar compilation progress in the test log.
  Reviewers: davidn
diff: @@ -311,7 +311,14 @@ StatusOr<std::unique_ptr<Executable>> PoplarCompiler::RunBackend(
  VLOG(1) << "Compile engine " << module->name();
  auto opts = poplarExecutor->GetOptionsFlags();
  - engine.reset(new poplar::Engine(graph, progs, opts));
  + auto progress_logging = [](int progress, int total) {
  + float progress_percent =
  + std::floor(100.0f * static_cast<float>(progress) /
  + static_cast<float>(total));
  + VLOG(1) << "Poplar compilation " << progress_percent << "% complete";
  + };
  +
  + engine.reset(new poplar::Engine(graph, progs, opts, progress_logging));
  } catch (std::logic_error e) {
  return tensorflow::errors::Unknown(
  StrCat("[Poplar Engine] ", e.what()));
  