code refactor for venv (thu-ml#179)

- Refacor code to remove duplicate code

- Enable async simulation for all vector envs

- Remove `collector.close` and rename `VectorEnv` to `DummyVectorEnv`

The abstraction of vector env changed.

Prior to this pr, each vector env is almost independent.

After this pr, each env is wrapped into a worker, and vector envs differ with their worker type. In fact, users can just use `BaseVectorEnv` with different workers, I keep `SubprocVectorEnv`, `ShmemVectorEnv` for backward compatibility.

Co-authored-by: n+e <463003665@qq.com>
Co-authored-by: magicly <magicly007@gmail.com>

docs fix and v0.2.5 (thu-ml#156)

* pre

* update docs

* update docs

* $ in bash

* size -> hidden_layer_size

* doctest

* doctest again

* filter a warning

* fix bug

* fix examples

* test fail

* test succ

Improve collector (thu-ml#125)

* remove multibuf

* reward_metric

* make fileds with empty Batch rather than None after reset

* many fixes and refactor
Co-authored-by: Trinkle23897 <463003665@qq.com>

move atari wrapper to examples and publish v0.2.4 (thu-ml#124)

* move atari wrapper to examples

* consistency

* change drqn seed since it is quite unstable in current seed

* minor fix

* 0.2.4

Fix log_prob and PPO dual_clip (thu-ml#49)

* Added DiagGaussian to fix log_probg

* Disable PPO dual_clip

fix pdqn

first version with full documentation