[BugFix] Raise exception in PixelObservationWrapper if env.render_mode is not specified

[vmoens]

Description

In 8e812e1 we added a warning when render_mode was not specified, but in 0.26 this will not work anymore.
Hence we require that env.render_mode is not None for PixelObservationWrapper to be used.

Behaviour ante

from gym.wrappers.pixel_observation import PixelObservationWrapper
import gym
base_env = gym. Make("Pendulum-v1") # no error
env = PixelObservationWrapper(env) # no error
env.reset() # error

With the error

~/venv/rl/lib/python3.8/site-packages/gym/utils/passive_env_checker.py in env_render_passive_checker(env, *args, **kwargs)
    314             )
    315 
--> 316     result = env.render(*args, **kwargs)
    317 
    318     # TODO: Check that the result is correct

TypeError: render() got an unexpected keyword argument 'mode'

Behaviour post

An error is raised as soon as PixelObservationWrapper is instantiated

from gym.wrappers.pixel_observation import PixelObservationWrapper
import gym
base_env = gym.make("Pendulum-v1") # no error
env = PixelObservationWrapper(env) # error

Solution

from gym.wrappers.pixel_observation import PixelObservationWrapper
import gym
base_env = gym. Make("Pendulum-v1", render_mode="human") # no error
env = PixelObservationWrapper(env) # no error
env.reset() # no error

[pseudo-rnd-thoughts]

We can reverted the meaning of the render_modes in v25 back to the previous meaning where rgb_array means single render obs and rgb_array_list will mean list of render obs

[pseudo-rnd-thoughts]

Please make the render mode change otherwise looks good

[vmoens]

Please make the render mode change otherwise looks good

Just made the change!