Code for "Grid R-CNN" (#810)

* Grid R-CNN

* add grid_rcnn_res50fpn2x config

* add assertion that grid_head should exist

* fix bugs and remove SharedFCBBoxHeadGrid

* remove the property with_grid

* format fixes for grad_head and add config dir

* move random_jitter to grid_head and some refactoring

* simplify the calculation of num_edges

* refactoring

* refactoring

* rename config files and add x101 config

* bug fix for inference

* remove random_jitter_single

* add readme of grid rcnn

* add bibtex of grid rcnn plus

* update work_dir