message: [XLA] Add unoptimized_hlo_proto dump to Service::BuildExecutables.
  With a recent change to Client::Execute, flag xla_dump_unoptimized_hlo_proto_to
  stops working. Fix this by handling the flag in Service::BuildExecutables.
diff: @@ -368,6 +368,7 @@ StatusOr<std::vector<std::unique_ptr<Executable>>> Service::BuildExecutables(
  const HloModuleProto* proto = module_protos[i];
  const HloModuleConfig& config = *module_configs[i];
  TF_ASSIGN_OR_RETURN(auto module, CreateModuleFromProto(*proto, config));
  + TF_RETURN_IF_ERROR(MaybeDumpUnoptimizedHloModule(*module));
  module_group->push_back(std::move(module));
  }
  