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>

CONTRIBUTING.md

@@ -12,7 +12,7 @@ possible.
    This should install hooks for running `black` and `clang-format` to ensure
    consistent style for C++ and python code.
  
-   You can also run the formatters manually as follows:
+   You can also run the formatters manually as follows on individual files:
  
      ```bash
      clang-format -i file.cpp
@@ -21,6 +21,16 @@ possible.
      ```bash
      black file.py
      ```
+
+     or,
+
+     ```bash
+     # single file
+     pre-commit run --files file1.py 
+
+     # specific files
+     pre-commit run --files file1.py file2.py
+     ```
  
    or run `pre-commit run --all-files` to check all files in the repo.