message: Add possibility to include default optimizers in custom optimizer list
diff: @@ -215,6 +215,16 @@ Status MetaOptimizer::InitializeCustomGraphOptimizers(
  TF_RETURN_IF_ERROR(custom_optimizer->Init(&optimizer_config));
  optimizers->push_back(std::move(custom_optimizer));
  } else {
  + // if there are no custom optimizers with given name, try to initalize a
  + // default optimizer. This way custom configurable optimizers can be
  + // mixed with default optimizers in any order.
  + auto optimizer = MakeNewOptimizer(optimizer_config.name());
  + if (optimizer) {
  + VLOG(2) << "Registered default graph optimizer: "
  + << optimizer_config.name();
  + optimizers->push_back(std::move(optimizer));
  + continue;
  + }
  VLOG(2) << "Can't register an optimizer by name: "
  << optimizer_config.name();
  }
  