Add rgb2gray and gray2rgb (open-mmlab#139)

* add rgb2gray and gray2rgb

* update version

* minor fix for the docstring

Update version to v0.2.13 (open-mmlab#117)

update to v0.2.12 (open-mmlab#104)

update version to v0.2.11 (open-mmlab#98)

Fix the compiling bug when using `pip install mmcv` (open-mmlab#88)

* rename flowWarp to FlowWarp

* fix the compiling bug when using pip install

* update version to v0.2.10

Improve the documentation (open-mmlab#85)

* fix doc generation

* add docs for optical flow

* fix typo

Merge pull request open-mmlab#70 from hellock/master

Update to v0.2.8

Merge pull request open-mmlab#63 from hellock/master

Add jhu gn model urls and update to v0.2.7

Update version to 0.2.6 (open-mmlab#54)

* fix the time estimation when resuming from a checkpoint

* fix the time estimation when resuming from a checkpoint

* support str type for loggers

* fix yaml dump tests

* update version to v0.2.6

update to v0.2.5