Configuration-based use of HF hub-hosted datasets for training (#701)

* Add hf_dataset configuration for using HF hub-hosted datasets for (Q)LoRA training

* Pre-commit formatting

* Fix YAML config example

* Print DS info

* Include name

* Add hf_dataset parameter default

* Remove TextHFDataset and CompletionsHFDataset and use Dataset and CompletionsDataset instead, adding a text_key constructor argument to the former (and changing it to work with a provided data structure instead of just from a JSON file), and prompt_key and completion_key arguments to the latter with defaults for backwards compatibility.

* nits

* update docs

---------

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

llms/setup.py

@@ -26,6 +26,9 @@ setup(
     install_requires=requirements,
     packages=["mlx_lm", "mlx_lm.models", "mlx_lm.tuner"],
     python_requires=">=3.8",
+    extras_require={
+        "testing": ["datasets"],
+    },
     entry_points={
         "console_scripts": [
             "mlx_lm.convert = mlx_lm.convert:main",