fix(dataset): Use provided episode_data in save_episode (#1740)

The 'episode_data' parameter was previously ignored, causing an error if provided. This change ensures it is correctly used, which allows for asynchronous episode saving by passing a copy of the episode buffer, preventing conflicts with the main data collection loop.

src/lerobot/datasets/lerobot_dataset.py

@@ -825,6 +825,8 @@ class LeRobotDataset(torch.utils.data.Dataset):
         """
         if not episode_data:
             episode_buffer = self.episode_buffer
+        else:
+            episode_buffer = episode_data
 
         validate_episode_buffer(episode_buffer, self.meta.total_episodes, self.features)