message: Add Pack to operator_properties.
  Fixes
diff: @@ -286,6 +286,12 @@ OperatorProperty GetOperatorProperty(const ModelT* model, int subgraph_index,
  property.outputs = {{0, {}}};
  property.version = 2;
  break;
  + case BuiltinOperator_PACK:
  + property.arbitrary_inputs = true;
  + property.outputs = {{0, {}}};
  + property.restrict_same_input_output_scale = true;
  + property.version = 2;
  + break;
  case BuiltinOperator_PAD:
  case BuiltinOperator_PADV2:
  property.inputs = {{0, {}}};
  