message: Fix bug in AddCallbackForEvent where we are using invalid events
diff: @@ -768,8 +768,7 @@ class PodTpuDriver : public TpuDriver {
  } else {
  fn(event_status->second);
  }
  - }
  -
  + } else {
  if (event->second->underlying_event != nullptr &&
  event->second->underlying_event.use_count() != 0) {
  event->second->underlying_event->AddCallback(fn);
  @@ -777,6 +776,7 @@ class PodTpuDriver : public TpuDriver {
  event->second->callbacks.push_back(std::move(fn));
  }
  }
  + }
  xla::Status GetCompiledProgramShape(int64_t op_id,
  xla::ProgramShapeProto* program_shape)
  