text: Title: Validation error on serializer used only for responses
  Body: Note: This may be what https://github.com/axnsan12/drf-yasg/issues/51 was trying to get at...
  
  Trying to do this, but getting a validation error ("... 'ssv': "Unresolvable JSON pointer: 'definitions/Detail'")
  ```
  class DetailSerializer(serializers.Serializer):
      detail = serializers.CharField()
  
  ```
  
  ```
  class ManufacturerViewSet(viewsets.ModelViewSet):
  
      serializer_class = ManufacturerSerializer
      model = Manufacturer
      queryset = Manufacturer.objects.all()
  
      @swagger_auto_schema(responses={404: openapi.Response("Not found or Not accessible", DetailSerializer,
                                                            examples={
                                                                'Not found': DetailSerializer({'detail':'Not found'}).data,
                                                                'Not accessible': DetailSerializer({'detail':'Not accessible'}).data,
                                                            },
                                                            )})
      def retrieve(self, request, *args, **kwargs):
          return super().retrieve(self, request, *args, **kwargs)
  
  ```
  
  However, if I add the serializer to recognized model, it does work, e.g.,
  
  ```
  class ManufacturerSerializer(serializers.ModelSerializer):
  
      status = DetailSerializer(many=False)
  
      class Meta:
          model = Manufacturer
          fields = '__all__'
  
  ```
  
  Full text of validation error...
  
  ```
  {'flex': "'paths':\n"
           "    - '/manufacturers/{id}/':\n"
           "        - 'get':\n"
           "            - 'responses':\n"
           "                - '404':\n"
           "                    - 'referenceObject':\n"
           "                        - 'additionalProperties':\n"
           '                            - "When `additionalProperties` is False, '
           'no unspecified properties are allowed. The following unspecified '
           "properties were found:\\n\\t`{'headers', 'description', 'examples', "
           '\'schema\'}`"\n'
           "                        - 'required':\n"
           "                            - '$ref':\n"
           "                                - 'This value is required'\n"
           "                    - 'responseObject':\n"
           "                        - 'schema':\n"
           "                            - '$ref':\n"
           "                                - 'The $ref `#/definitions/Detail` "
           "was not found in the schema'",
   'ssv': "Unresolvable JSON pointer: 'definitions/Detail'"}
  ```