Check that the code implementation is accurate and reasonable

- [x] check and fix C51 [deaab73]
- [x] check qrdqn [deaab73]
- [ ] check iqn
- [ ] check and fix Rainbow
- [ ] check on-policy buffer sampling
- [ ] check function `discounted_sum`
- [ ] check function `calculate_td_error`
- [ ] checke whether works well when training with visual input
- [ ] fix TRPO that step_size sometime be `nan`
- [ ] check `vdn` and `qmix`