Reimplement "Mask Scoring R-CNN" (#621)

* add ms-rcnn

* fix config

* fix bug in test

* fix bug in test, det_labels should plus 1

* add area ratio

* speed up area ratio calculation

* delete caffe style config

* refactor code, bbox and segm eval using different score

* fix bug in distevalhook

* update README and config

* update x101-64x4d

* refactor code

* update upstream

* format

* update r50 and r101 2x performance

* update ms-rcnn-x101-64x4d-2x performance

* move simple_mask_test, add docstring and comment

* delete TODO in mask iou target

* add detail comment to mask iou target

* modify comment of mask iou target

* move mask iou target to mask iou head

* merge mask_iou_target and get_target

* add MSELoss for MaskIoUHead and rename mask_iou to fc_mask_iou

* refactoring

* fix bug in test

* fix bug when cropping mask inside proposal

* update readme

* update bibtex

* remove space in bibtex