refactor(datasets): replace untyped dict with typed DatasetInfo dataclass (#3472)

* refactor(datasets): replace untyped dict with typed DatasetInfo dataclass

Introduce typed DatasetInfo dataclass to replace untyped dict representation of info.json.

Changes:
- Add DatasetInfo dataclass with explicit fields and validation
- Implement __post_init__ for shape conversion (list ↔ tuple)
- Add dict-style compatibility layer (__getitem__, __setitem__, .get())
- Add from_dict() and to_dict() for JSON serialization
- Update io_utils to use load_info/write_info with DatasetInfo
- Update dataset utilities and metadata to use attribute access
- Remove aggregate.py dict-style field access
- Add tests fixture support for DatasetInfo

Benefits:
- Type safety with IDE auto-completion
- Validation at construction time
- Explicit schema documentation

* fix pre-commit

* update docstring inside DatasetInfo.from_dict()

* sorts the unknown to have deterministic output

Signed-off-by: Maxime Ellerbach <maxime@ellerbach.net>

* refactoring the last few old fieds


* fix crop dataset roi type mismatch


* use consistantly int for data and video_files_size_in_mb

---------

Signed-off-by: Maxime Ellerbach <maxime@ellerbach.net>
Co-authored-by: jjolla93 <jjolla93@gmail.com>

src/lerobot/datasets/multi_dataset.py

@@ -123,7 +123,7 @@ class MultiLeRobotDataset(torch.utils.data.Dataset):
 
         NOTE: Fow now, this relies on a check in __init__ to make sure all sub-datasets have the same info.
         """
-        return self._datasets[0].meta.info["fps"]
+        return self._datasets[0].meta.info.fps
 
     @property
     def video(self) -> bool:
@@ -133,7 +133,7 @@ class MultiLeRobotDataset(torch.utils.data.Dataset):
 
         NOTE: Fow now, this relies on a check in __init__ to make sure all sub-datasets have the same info.
         """
-        return self._datasets[0].meta.info.get("video", False)
+        return len(self._datasets[0].meta.video_keys) > 0
 
     @property
     def features(self) -> datasets.Features: