feat: add update_config functionality (#531)

* feat: add `update_config` finctionality

- sorts the config for better readability
- updates "_name_or_path" key in config with upload_repo
- sets indentation of 4 spaces
- allows adding other key-value pairs via kwargs
- reduces code duplication
- standardizes config-update across mlx-lm

* feat: standardize updating config

Impactes:
- fuse.py
- merge.py

* update formatting

* remove commented out code

* update func: update_config to save_config

- drop kwards
- rename func as save_config
- incorporate review suggestions

* update func: save_config

- ensure only config-saving functionality
- function oes not return config as a dict anymore
- added review suggestions

* fixed formatting

* update formatting instruction in contribution guide

* nits

---------

Co-authored-by: Awni Hannun <awni@apple.com>

llms/mlx_lm/fuse.py

@@ -9,7 +9,13 @@ from mlx.utils import tree_flatten, tree_unflatten
 
 from .tuner.lora import LoRALinear
 from .tuner.utils import apply_lora_layers, dequantize
-from .utils import fetch_from_hub, get_model_path, save_weights, upload_to_hub
+from .utils import (
+    fetch_from_hub,
+    get_model_path,
+    save_config,
+    save_weights,
+    upload_to_hub,
+)
 
 
 def parse_arguments() -> argparse.Namespace:
@@ -87,8 +93,7 @@ def main() -> None:
     if args.de_quantize:
         config.pop("quantization", None)
 
-    with open(save_path / "config.json", "w") as fid:
-        json.dump(config, fid, indent=4)
+    save_config(config, config_path=save_path / "config.json")
 
     if args.upload_repo is not None:
         hf_path = args.hf_path or (