Add type hints to action/obs spaces:

  * Should `BranchingPolicy` support also `MultiDiscrete` action spaces?

test/discrete/test_bdq.py

@@ -52,7 +52,10 @@ def test_bdq(args: argparse.Namespace = get_args()) -> None:
     env = gym.make(args.task)
     env = ContinuousToDiscrete(env, args.action_per_branch)
 
-    args.state_shape = env.observation_space.shape or env.observation_space.n
+    if isinstance(env.observation_space, gym.spaces.Box):
+        args.state_shape = env.observation_space.shape
+    elif isinstance(env.observation_space, gym.spaces.Discrete):
+        args.state_shape = int(env.observation_space.n)
     assert isinstance(env.action_space, gym.spaces.MultiDiscrete)
     args.num_branches = env.action_space.shape[0]
 
@@ -100,7 +103,7 @@ def test_bdq(args: argparse.Namespace = get_args()) -> None:
         model=net,
         optim=optim,
         discount_factor=args.gamma,
-        action_space=env.action_space,
+        action_space=env.action_space,  # type: ignore[arg-type]  # TODO: should BranchingPolicy support also `MultiDiscrete` action spaces?
         target_update_freq=args.target_update_freq,
     )
     # collector