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 pdqn

first version with full documentation