Decouple `LogEnvState`

[sash-a]

Having the LogWrapper and LogEnvState means that we often have to do state.env_state which is quite confusing. So just tracking the logging metrics separately in it's own state would make this cleaner and more maintainable

Currently it's done like this:

@dataclass
class LogEnvState:
    """State of the `LogWrapper`."""

    env_state: State
    episode_returns: chex.Numeric
    episode_lengths: chex.Numeric
    # Information about the episode return and length for logging purposes.
    episode_return_info: chex.Numeric
    episode_length_info: chex.Numeric

I'm proposing this:

@dataclass
class LoggerState:
    episode_returns: chex.Numeric
    episode_lengths: chex.Numeric
    # Information about the episode return and length for logging purposes.
    episode_return_info: chex.Numeric
    episode_length_info: chex.Numeric

and then we create methods to reset and step this state and the env state is reset and steped separately

Have chatted to @OmaymaMahjoub about this and we agree it should be cleaner