message: [tf.data] When prefetch thread finishes due to cancellation, notify the cond_var_.
  This ensures that the consuming thread will not wait forever in GetNextInternal() for the next prefetched element.
diff: @@ -385,6 +385,8 @@ class PrefetchDatasetOp::Dataset : public DatasetBase {
  }
  if (cancellation_manager_.IsCancelled()) {
  + prefetch_thread_finished_ = true;
  + cond_var_->notify_all();
  return;
  }
  }
  