* add isort config

* use isort to sort imports

* add isort to travis

* Require an operation(Save or Show the results) when testing

* using parentheses

* Warning when skip image

* added parameter skip_img_without_anno for dataset

* print filename when warning

* use registry to manage datasets

* bug fix for concat dataset

* update documentation to fit the new api

* add HybridTaskCascade

* add configs for other backbones

* add keep_ratio argument for segmap transform

* add readme for HTC

* fix linting errors

* split assign and sampling as in Cascade R-CNN

* remove unused imports

* add a large model

* update htc