- [ ] I have marked all applicable categories:
    + [ ] exception-raising fix
    + [ ] algorithm implementation fix
    + [ ] documentation modification
    + [ ] new feature
- [ ] If applicable, I have mentioned the relevant/related issue(s)

Less important but also useful:

- [ ] I have visited the [source website](https://github.com/thu-ml/tianshou)
- [ ] I have searched through the [issue tracker](https://github.com/thu-ml/tianshou/issues) for duplicates
- [ ] I have mentioned version numbers, operating system and environment, where applicable:
  ```python
  import tianshou, torch, sys
  print(tianshou.__version__, torch.__version__, sys.version, sys.platform)
  ```