def on_fit_start(self, trainer: "pl.Trainer", pl_module: "pl.LightningModule") -> None:
        """
        Raises:
            MisconfigurationException:
                If LightningModule has no nn.Module `backbone` attribute.
        """
        if hasattr(pl_module, "backbone") and isinstance(pl_module.backbone, Module):
            return super().on_fit_start(trainer, pl_module)
        raise MisconfigurationException("The LightningModule should have a nn.Module `backbone` attribute")

        Raises:
            MisconfigurationException:
                If LightningModule has no nn.Module `backbone` attribute.
        
src/lightning/pytorch/callbacks/finetuning.py