Update atari_preprocessing.py (#3105)

gym/wrappers/atari_preprocessing.py

@@ -98,7 +98,6 @@ def __init__(
                 np.empty(env.observation_space.shape, dtype=np.uint8),
             ]
 
-        self.ale = env.unwrapped.ale
         self.lives = 0
         self.game_over = False
 
@@ -111,6 +110,11 @@ def __init__(
         self.observation_space = Box(
             low=_low, high=_high, shape=_shape, dtype=_obs_dtype
         )
+
+    @property
+    def ale(self):
+        """Make ale as a class property to avoid serialization error"""
+        return self.env.unwrapped.ale
 
     def step(self, action):
         """Applies the preprocessing for an :meth:`env.step`."""