move video dir assertion

lerobot/scripts/eval.py

@@ -233,6 +233,9 @@ def eval_policy(
     Returns:
         Dictionary with metrics and data regarding the rollouts.
     """
+    if max_episodes_rendered > 0 and not videos_dir:
+        raise ValueError("If max_episodes_rendered > 0, videos_dir must be provided.")
+
     assert isinstance(policy, Policy)
     start = time.time()
     policy.eval()
@@ -355,7 +358,6 @@ def eval_policy(
         # Maybe render video for visualization.
         if max_episodes_rendered > 0 and len(ep_frames) > 0:
             batch_stacked_frames = np.stack(ep_frames, axis=1)  # (b, t, *)
-            assert isinstance(videos_dir, Path)
             for stacked_frames, done_index in zip(
                 batch_stacked_frames, done_indices.flatten().tolist(), strict=False
             ):