Finally I got answer myself for polymorphic relation to save in repository.

In create function--

  if($this->validator->isValid($inputs){
      // first store in temp model. $this model will be destroyed.
       $temp_model = $this->model->create($inputs);
      //then Depency Injection comein for polymorphic model
       $this->polymorphicmodel->create($subinputs):
       // link with polymorphic relation with $TEMP_MODEL not this->model. 
       $this->polymorphicmodel->modelable()->save($temp_model);
       return true;
    }
   return false;

really appreciate advanced modification or suggestion for better approached.