[Python Server] Remove abstractmethod notation for add_registered_method_handlers (#36684)

Remove `@abc.abstractmethod` so we don't break backward compatibility.

Fix: https://github.com/grpc/grpc/issues/36683

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Closes #36684

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36684 from XuanWang-Amos:remove_abstractmethod_for_registered_method f748fa6e7a
PiperOrigin-RevId: 636186951

src/python/grpcio/grpc/__init__.py

@@ -1454,7 +1454,6 @@ class Server(abc.ABC):
         """
         raise NotImplementedError()
 
-    @abc.abstractmethod
     def add_registered_method_handlers(self, service_name, method_handlers):
         """Registers GenericRpcHandlers with this Server.
 
@@ -1468,7 +1467,6 @@ class Server(abc.ABC):
           method_handlers: A dictionary that maps method names to corresponding
             RpcMethodHandler.
         """
-        raise NotImplementedError()
 
     @abc.abstractmethod
     def add_insecure_port(self, address):

src/python/grpcio/grpc/aio/_base_server.py

@@ -136,7 +136,6 @@ class Server(abc.ABC):
           A bool indicates if the operation times out.
         """
 
-    @abc.abstractmethod
     def add_registered_method_handlers(self, service_name, method_handlers):
         """Registers GenericRpcHandlers with this Server.
 
@@ -147,7 +146,6 @@ class Server(abc.ABC):
           method_handlers: A dictionary that maps method names to corresponding
             RpcMethodHandler.
         """
-        raise NotImplementedError()
 
 
 # pylint: disable=too-many-public-methods