[Refactor] Using mmcv transformer bricks to refactor vit. (open-mmlab#571)

* [Refactor] Using mmcv bricks to refactor vit

* Follow the vit code structure from mmclassification

* Add MMCV install into CI system.

* Add  to 'Install MMCV' CI item

* Add 'Install MMCV_CPU' and 'Install MMCV_GPU CI' items

* Fix & Add

1. Fix low code coverage of vit.py;

2. Remove HybirdEmbed;

3. Fix doc string of VisionTransformer;

* Add helpers unit test.

* Add converter to convert vit pretrain weights from timm style to mmcls style.

* Clean some rebundant code and refactor init

1. Use timm style init_weights;

2. Remove to_xtuple and trunc_norm_;

* Add comments for VisionTransformer.init_weights()

* Add arg: pretrain_style to choose timm or mmcls vit pretrain weights.

Author: clownrat6