How to support multi-agent reinforcement learning

I have marked all applicable categories:

• exception-raising bug
• RL algorithm bug
• documentation request (i.e. "X is missing from the documentation.")
• new feature request

I have visited the source website, and in particular read the known issues

I have searched through the issue tracker and issue categories for duplicates

I have mentioned version numbers, operating system and environment, where applicable:

import tianshou, torch, sys
print(tianshou.__version__, torch.__version__, sys.version, sys.platform)