Algorithm Implementation

1. Generalized Advantage Estimation (GAE);
2. Update PPO algorithm with arXiv:1811.02553 and arXiv:1912.09729;
3. Vanilla Imitation Learning (BC & DA, with continuous/discrete action space);
4. Prioritized DQN;
5. RNN-style policy network;
6. Fix SAC with torch==1.5.0

API change

1. change __call__ to forward in policy;
2. Add save_fn in trainer;
3. Add __repr__ in tianshou.data, e.g. print(buffer)

First version with full documentation.
Support algorithms: DQN/VPG/A2C/DDPG/PPO/TD3/SAC