add a asdict for dataclass model_config

[xliu0105]

The get_model_config function in tuners_utils.py does not handle model_config of dataclass data type, because some models (such as the lerobot model) have config attributes, but the dataclass type cannot be converted into a dictionary in the get_model_config function, resulting in an error when using .get in the _get_tied_target_modules function.

I fixed this problem. Make peft adapt to a wider range of model types.

[githubnemo]

Yes, I planned to do this as a follow-up to a potential discussion of huggingface/lerobot#1411 but I think it doesn't hurt to support dataclasses as model config objects, especially since the change is not that big.

Could you just add a unit test for this case in tests/test_config.py? We could simply test that get_model_config returns a dictionary given a model with a dataclass config.