message: Remove elem_type from tensorflow Pack export, tensorflow doesn't like graphs that use
  this attribute in Pack.
diff: @@ -1257,8 +1257,6 @@ void ConvertPackOperator(const Model& model, const PackOperator& src_op,
  for (const auto& input : src_op.inputs) {
  *pack_op->add_input() = input;
  }
  - (*pack_op->mutable_attr())["elem_type"].set_type(
  - GetTensorFlowDataType(model, src_op.outputs[0]));
  (*pack_op->mutable_attr())["axis"].set_i(src_op.axis);
  (*pack_op->mutable_attr())["N"].set_i(src_op.inputs.size());
  (*pack_op->mutable_attr())["T"].set_type(GetTensorFlowDataType(src_op.dtype));
  