message: Allow back propagation across RELU. This is used for inference engines
  that do not fuse activation functions.
diff: @@ -110,6 +110,13 @@ bool DoesOpBlockBackwardPropagation(const Operator& op) {
  case OperatorType::kSelect:
  case OperatorType::kTile:
  // Reshapes and transposes don't change values.
  + case OperatorType::kRelu:
  + case OperatorType::kRelu1:
  + case OperatorType::kRelu6:
  + // Relus only clamp the output. If min/max of parent is unknown, just
  + // prop the range backward. This only happens for cases where activations
  + // are not fused to avoid a default being set on the RELU input and
  + // propagating forward to the RELU output.
  return false;
  default:
  return true;
  