[BUG] VecNorm.to_observation_norm broken for multiple keys

Describe the bug

The VecNorm transform produces a bug when calling the to_observation_norm method with multiple keys

To Reproduce

gym_env = GymEnv("MountainCarContinuous-v0", device='cpu')
transformed_env = TransformedEnv(gym_env, VecNorm(in_keys=["observation", "reward"]))
transformed_env.rollout(100)
transformed_env.transform[0].to_observation_norm()

Expected behavior

Should return ObservationNorm but gives following error:

...
out += ObservationNorm
TypeError: 'type' object is not iterable

Reason and Possible fixes

Using the VecNorm transform with only one key works - so to do it for multiple keys one can just use it repeatedly with single keys.

Checklist

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