v0.20.0: Hotfix for PPO with un-normalized env, `net_arch` support for PPO, additional fixes

What's Changed

• Update PPO to support net_arch, and additional fixes by @araffin in #65
• fixed entropy coeff wrongly logged for SAC and derivatives.
• fixed PPO predict() for env that were not normalized (action spaces with limits != [-1, 1])
• PPO now logs the standard deviation

Full Changelog: v0.19.0...v0.20.0