message: Fix bug in NCCL broadcast wrapper
  The recvbuf passed to ncclBroadcast must not be nullptr. This issue
  arises if the source node of the broadcast has no output.
diff: @@ -703,6 +703,9 @@ void NcclManager::LoopKernelLaunches(NcclStream* nccl_stream) {
  if (p->output) {
  recvbuff = const_cast<char*>(p->output->tensor_data().data());
  num_elements = p->output->NumElements();
  + } else {
  + // Operate in-place if no output (for the src node).
  + recvbuff = const_cast<void*>(sendbuff);
  }
  if (num_elements < 0) {
  p->done_callback(errors::Internal(
  