class_docstr: Base FastAPI server. Implement this abstract class in-case you want to build a fastapi-based server by
  implementing the `app` property. This property should return a fastapi app. The base Gateway will handle starting
  a server and serving the application using that server.
docstr: Get a FastAPI app
docstr_tok: ["Get","a","FastAPI","app"]
code: def app(self):
      """Get a FastAPI app"""
      ...
  
imports: import logging
  import os
  from abc import abstractmethod
  from typing import TYPE_CHECKING, Optional
  from jina._docarray import docarray_v2
  from jina.importer import ImportExtensions
  from jina.serve.runtimes.servers import BaseServer
label_desc: Use image node_id 2 for calling the FastAPIBaseServer obj's underlying member method code with example usage: obj.app() without return types