Updates: Support the latest Atari environment and state entropy maximization-based exploration.

[yuanmingqi]

• Update for supporting the latest Atari environment
  Tested using the following dependences:
  stable-baselines3==1.5.0
  gym==0.21.0
  ale-py==0.7.4

• Update for supporting state entropy maximization-based exploration
  Intrinsic rewards can improve the exploration when handling complex environments with high-dimensional observations. Thus I added the following module entitled "State entropy maximization with random encoders for efficient exploration (RE3)". Since RE3 requires no auxiliary models, it won't decrease the computational efficiency. Use --use--sem to invoke it!