message: provide default implementation for future refactoring..
diff: @@ -179,9 +179,9 @@ class CuptiTraceCollector {
  virtual void Flush() = 0;
  // Consumer side functions (i.e. called by GPU tracer);
  - virtual void Export(StepStats* step_stats) = 0;
  - virtual void Export(XSpace* space, uint64 end_gpu_ns) = 0;
  - virtual std::string ReportNumEventsIfDropped() = 0;
  + virtual void Export(StepStats* step_stats) {}
  + virtual void Export(XSpace* space, uint64 end_gpu_ns) {}
  + virtual std::string ReportNumEventsIfDropped() { return ""; }
  AnnotationMap* annotation_map() { return &annotation_map_; }
  