refactor(utils): enhance task handling in add_envs_task function

- Improved the `add_envs_task` function to validate the output of `task_description` and `task` calls, ensuring they return lists of strings.
- Removed the use of `else` statement for environments without language instructions, simplifying the logic and enhancing readability.
- Streamlined the observation dictionary handling by ensuring consistent data types for task attributes.

src/lerobot/scripts/eval.py

@@ -155,7 +155,6 @@ def rollout(
     while not np.all(done):
         # Numpy array to tensor and changing dictionary keys to LeRobot policy format.
         observation = preprocess_observation(observation)
-        observation = preprocessor(observation)
         if return_observations:
             all_observations.append(deepcopy(observation))