feat: Add root directory option for dataset configuration (#765)

Co-authored-by: Simon Alibert <75076266+aliberts@users.noreply.github.com>

lerobot/configs/default.py

@@ -29,6 +29,8 @@ class DatasetConfig:
     # "dataset_index" into the returned item. The index mapping is made according to the order in which the
     # datsets are provided.
     repo_id: str
+    # Root directory where the dataset will be stored (e.g. 'dataset/path').
+    root: str | None = None
     episodes: list[int] | None = None
     image_transforms: ImageTransformsConfig = field(default_factory=ImageTransformsConfig)
     revision: str | None = None