* [Squash] Refator ViT (from #295)
* Use base variable to simplify auto_aug setting
* Use common PatchEmbed, remove HybridEmbed and refactor ViT init
structure.
* Add `output_cls_token` option and change the output format of ViT and
input format of ViT head.
* Update unit tests and add test for `output_cls_token`.
* Support out_indices.
* Standardize config files
* Support resize position embedding.
* Add readme file of vit
* Rename config file
* Improve docs about ViT.
* Update docstring
* Use local version `MultiheadAttention` instead of mmcv version.
* Fix MultiheadAttention
* Support `qk_scale` argument in `MultiheadAttention`
* Improve docs and change `layer_cfg` to `layer_cfgs` and support
sequence.
* Use init_cfg to init Linear layer in VisionTransformerHead
* update metafile
* Update checkpoints and configs
* Imporve docstring.
* Update README
* Revert GAP modification.
Ma Zerun