text: Title: Swagger throws Internal Server Error when using reverse relationships in serializer
  Body: **error:**
   File "/usr/local/lib/python3.6/dist-packages/drf_yasg/inspectors/field.py", line 85, in field_to_swagger_object
      raise SwaggerGenerationError("cannot instantiate nested serializer as " + swagger_object_type.__name__)
  drf_yasg.errors.SwaggerGenerationError: cannot instantiate nested serializer as Parameter
  
  ```
  //models
  class ModelA(BaseModel):
   ...some fields...
      class Meta:
          db_table = 'model_a'
  
  class ModelB(BaseModel):
      model_a_id = models.ForeignKey(ModelA, on_delete=models.CASCADE, related_name='model_b_info')
      ...some fields....
      class Meta:
          db_table = 'model_b'
  
  //serializers
  
  class ModelBReadSerializer(serializers.ModelSerializer):
      some_id = someSerializer(many=True, read_only=True)
      class Meta:
          model = ModelB
          fields = "__all__"
  
  class ModelASerializer(serializers.ModelSerializer):
      model_b_info = ModelBReadSerializer(many=True,read_only=True)
      ...more fields...
          class Meta:
              model = ModelA
              fields = "__all__"
  ```
  