Fix converting list of LazyFrames to ndarray (#907)

Author: batist73

baselines/common/atari_wrappers.py

@@ -254,6 +254,13 @@ def __len__(self):
         return len(self._force())
 
     def __getitem__(self, i):
+        return self._force()[i]
+
+    def count(self):
+        frames = self._force()
+        return frames.shape[frames.ndim - 1]
+
+    def frame(self, i):
         return self._force()[..., i]
 
 def make_atari(env_id, max_episode_steps=None):