fix(depth unit): adding input depth unit storage in the dataset metadata (#3899)

* fix(depth unit): storing raw depth units in the dataset metadata for correct depth statistics and depth raw frames handling. The unit is stored as a string ("m","mm") under "depth_unit" at the same level as "is_depth_map". Unit is inferred from the depth frame type.

* feat(raw frame unit): adapting dataset reader so that raw depth frames are scaled according to the requested unit

* feat(stats units): rescaling stats when loading a dataset so that the stats are given in the requested unit

* tests(unit): adapting and extending depth tests to units manipulations

* chore(format): formating code

* feat(warning): adding a warning when depth unit is not specified in the dataset

* chore(infer_depth_unit): moving the depth unit inference utility in a more accessible location

* feat(rerun unit): adding correct depth unit display for rerun (foxglove does not support units yet)

* feat(unit getter): adding a proper output_depth_unit getter to LeRobotDataset for cleaner integration

* fix(streaming dataset): extending support for depth units to streaming datasets

* test(rerun): fixing rerun tests

tests/utils/test_rerun_visualization.py

@@ -50,8 +50,9 @@ def mock_rerun(monkeypatch):
             return self
 
     class DummyDepthImage:
-        def __init__(self, arr, colormap=None):
+        def __init__(self, arr, meter=None, colormap=None):
             self.arr = arr
+            self.meter = meter
             self.colormap = colormap
 
     def dummy_log(key, obj=None, **kwargs):