code: def instance(cls, name):
          
          if not cls._instance.get(name):
              model_name = name.split('.')
              ins_name = '.'.join(
                  ['models', model_name[0], 'model', model_name[1]])
              cls._instance[name] = cls.import_model(ins_name)()
  
          return cls._instance[name]
docs: Instantiate a model class according to import path
          args:
              name: class import path like `user.User`
          return:
              model instance