* Allowing validation dataset for computing validation loss

* added validation dataset

* fixing datasets errors

* Fixing linting errors

* add isort config

* use isort to sort imports

* add isort to travis

* Add option to use linear scaling rule

* Ensure pep8 conformity

* use registry to manage datasets

* bug fix for concat dataset

* update documentation to fit the new api

* Set LOCAL_RANK in script.
Fix multiple checkpoint download bug

Signed-off-by: innerlee <363664470@qq.com>

* beautify

Signed-off-by: innerlee <363664470@qq.com>

* Reformat use yapf 0.27.0

* Add yapf style file

* update the high-level inference api

* save classes in meta data and use it for visualization