auto block_arg_num = GetBlockArgNum(input);
  if (block_arg_num.has_value()) return block_arg_num;

  Value next_input = input;
  auto pad_op = dyn_cast_or_null<TF::PadOp>(next_input.getDefiningOp());
  auto cast_op = dyn_cast_or_null<TF::CastOp>(next_input.getDefiningOp());

  while (pad_op || cast_op) {
    if (pad_op) {
      auto block_arg_num = GetBlockArgNum(pad_op.getInput());