message: it seems that callback can be triggered after unsubscribe (bug in cupti). Defend ourselves from that.
  probably due to not calling cuptiFinalize in CUDA 10.1. We will evaluate that.
diff: @@ -755,6 +755,7 @@ Status CuptiTracer::Finalize() {
  Status CuptiTracer::HandleCallback(CUpti_CallbackDomain domain,
  CUpti_CallbackId cbid,
  const CUpti_CallbackData *callback_info) {
  + if (!api_tracing_enabled_) return Status::OK(); // already unsubscribed.
  if (domain != CUPTI_CB_DOMAIN_DRIVER_API) return Status::OK();
  if (callback_info->callbackSite == CUPTI_API_ENTER) {
  // Stash away the current Cupti timestamp into callback_info.
  