Loads episode_data_index and stats during dataset __init__ (#85)

Co-authored-by: Simon Alibert <75076266+aliberts@users.noreply.github.com>
Co-authored-by: Alexander Soare <alexander.soare159@gmail.com>

lerobot/common/envs/utils.py

@@ -15,8 +15,19 @@ def preprocess_observation(observation, transform=None):
 
     for imgkey, img in imgs.items():
         img = torch.from_numpy(img)
-        # convert to (b c h w) torch format
+
+        # sanity check that images are channel last
+        _, h, w, c = img.shape
+        assert c < h and c < w, f"expect channel first images, but instead {img.shape}"
+
+        # sanity check that images are uint8
+        assert img.dtype == torch.uint8, f"expect torch.uint8, but instead {img.dtype=}"
+
+        # convert to channel first of type float32 in range [0,1]
         img = einops.rearrange(img, "b h w c -> b c h w")
+        img = img.type(torch.float32)
+        img /= 255
+
         obs[imgkey] = img
 
     # TODO(rcadene): enable pixels only baseline with `obs_type="pixels"` in environment by removing requirement for "agent_pos"