    def gen_function(self) -> str:
        """call backend function"""
        target = backend.target.Target.current()
        func_key = "{target}.{op}.gen_function".format(
            target=target.name(), op=self._attrs["op"]
        )
        func = registry.get(func_key)
        return func(self._attrs)
call backend function
python/aitemplate/compiler/ops/tensor/size.py