Support for Discrete and Dict spaces in policy-based algorithms

[prabhasak]

Hello,

Thank you for this fantastic repo! I was hoping to use some of the implementations here for the MineRL competition. However, it seems policy-based algorithms like A2C,PPO, SAC, and fD do not support Dict and Discrete spaces, which is crucial for MineRL (and custom envs in general).

There are potential workarounds as @MrSyee was kind enough to point out, but it would be great if you could introduce minimal support in the near future!