* add norm_eval

* upgrade mmcv

* Use official DDP to implement MMDDP

* update mmcv requirement

* Update the version of MMCV

Co-authored-by: Kai Chen <chenkaidev@gmail.com>

* use git repo instead of pip wheels

* install pycocotools before building mmdet

* fix travis installation

* reorganize requirements, make albumentations optional

* fix flake8 error

* force older version of Pillow until torchvision is fixed

* make imagecorruptions optional and update INSTALL.dm

* update INSTALL.md

* Add note about pillow version

* Add build requirements to install instructions