Signals not working within classes

Describe the bug
You're unable to use signal decorators within classes.

To Reproduce

from tortoise.backends.base.client import BaseDBAsyncClient
from tortoise.signals import post_save
from typing import List, Optional, Type

from .models import Model


class Example:
    @post_save(Model)
    async def signal_post_save(
        self,
        sender: Type[Model],
        instance: Model,
        created: bool,
        using_db: Optional[BaseDBAsyncClient],
        update_fields: List[str],
    ) -> None:
        print(sender, instance, using_db, created, update_fields)

Start with the post_save decorator in tortoise.signals and trace how it handles the Example class method shown in the reproduction. Reproduce the failure with the provided Model and callback, then verify that signal decorators work within classes without breaking existing callback behavior.