docs: public void putFunctionForId(String id, DifferentialFunction function) {
          if (ops.containsKey(id) && ops.get(id).getOp() == null) {
              throw new ND4JIllegalStateException("Function by id already exists!");
          } else if (function instanceof SDVariable) {
              throw new ND4JIllegalStateException("Function must not be a variable!");
          }
  
          if(ops.containsKey(id)){
  
          } else {
              ops.put(id, SameDiffOp.builder().name(id).op(function).build());
          }
      }