Custom class handler missing BaseHandler

[swstack]

📚 The doc issue

I believe the docs for a custom class level entry point are missing the base-class BaseHandler. If i'm mistaken, please close this issue.

Link: https://github.com/pytorch/serve/blob/master/docs/custom_service.md#custom-handler-with-class-level-entry-point

Suggest a potential alternative/fix

Replace class ModelHandler(object): with class ModelHandler(BaseHandler):