is_terminated return type is unexpected in multi_threaded_env

[Graeme22]

MultiThreadEnv has a property called terminals which is a BitArray{1} containing the termination information for all of the wrapped environments.

However, the is_terminated function behaves in an unexpected way:

function RLBase.is_terminated(env::MultiThreadEnv)
    env.terminals .= is_terminated.(env.envs)
    env.terminals
end

The function implements the same is_terminated required for any AbstractEnv; however, instead of returning a Bool as the name and typical usage of the function would imply, it returns the env.terminals property which is a BitArray.

The problem with this is that it breaks any code that expects an is_terminated call to return a Bool, such as the StopAfterEpisode hook. I propose changing the function to the following:

function RLBase.is_terminated(env::MultiThreadEnv)
    env.terminals .= is_terminated.(env.envs)
    env.terminals == trues(size(env.terminals))
end

This way, the function will behave as expected in the majority of cases, and anyone who needs the termination status of individual wrapped envs can still access that with env.terminals.

[HenriDeh]

Sounds good to me