code: public SDVariable addVariable(SDVariable variable) {
          Preconditions.checkState(variable.getSameDiff() == this, "Samediff instance must be the same.");
  
          if (variables.containsKey(variable.getVarName()) && !variables.get(variable.getVarName()).getVariable().equals(variable)) {
              throw new IllegalArgumentException("Variable already found with variable opName " + variable.getVarName());
          }
  
          Preconditions.checkState(variable.getSameDiff() == this, "Same diff instance for variable must be the same!");
          variables.put(variable.getVarName(), Variable.builder().name(variable.getVarName()).variable(variable).build());
          return variable;
      }
docs: Add the specified variable to this SameDiff instance
  @param variable Variable to add