Add grid rcnn benchmark (#864)

* add grid benchmark

* minor update to readme

configs/grid_rcnn/README.md

@@ -22,12 +22,11 @@
 
 | Backbone    | Lr schd | Mem (GB) | Train time (s/iter) | Inf time (fps) | box AP | Download |
 |:-----------:|:-------:|:--------:|:-------------------:|:--------------:|:------:|:--------:|
-| R-50        | 2x      | 4.8      |                     |                | 40.3   | [model](https://open-mmlab.s3.ap-northeast-2.amazonaws.com/mmdetection/models/grid_rcnn/grid_rcnn_gn_head_r50_fpn_2x_20190619-5b29cf9d.pth) |
-| R-101       | 2x      | 6.7      |                     |                | 41.7   | [model](https://open-mmlab.s3.ap-northeast-2.amazonaws.com/mmdetection/models/grid_rcnn/grid_rcnn_gn_head_r101_fpn_2x_20190619-a4b61645.pth) |
-| X-101-32x4d | 2x      | 8.0      |                     |                | 43.0   | [model](https://open-mmlab.s3.ap-northeast-2.amazonaws.com/mmdetection/models/grid_rcnn/grid_rcnn_gn_head_x101_32x4d_fpn_2x_20190619-0bbfd87a.pth) |
-| X-101-64x4d | 2x      | 10.9     |                     |                | 43.1   | [model](https://open-mmlab.s3.ap-northeast-2.amazonaws.com/mmdetection/models/grid_rcnn/grid_rcnn_gn_head_x101_64x4d_fpn_2x_20190619-8f4e20bb.pth) |
+| R-50        | 2x      | 4.8      | 1.172               | 10.9           | 40.3   | [model](https://open-mmlab.s3.ap-northeast-2.amazonaws.com/mmdetection/models/grid_rcnn/grid_rcnn_gn_head_r50_fpn_2x_20190619-5b29cf9d.pth) |
+| R-101       | 2x      | 6.7      | 1.214               | 10.0           | 41.7   | [model](https://open-mmlab.s3.ap-northeast-2.amazonaws.com/mmdetection/models/grid_rcnn/grid_rcnn_gn_head_r101_fpn_2x_20190619-a4b61645.pth) |
+| X-101-32x4d | 2x      | 8.0      | 1.335               | 8.5            | 43.0   | [model](https://open-mmlab.s3.ap-northeast-2.amazonaws.com/mmdetection/models/grid_rcnn/grid_rcnn_gn_head_x101_32x4d_fpn_2x_20190619-0bbfd87a.pth) |
+| X-101-64x4d | 2x      | 10.9     | 1.753               | 6.4            | 43.1   | [model](https://open-mmlab.s3.ap-northeast-2.amazonaws.com/mmdetection/models/grid_rcnn/grid_rcnn_gn_head_x101_64x4d_fpn_2x_20190619-8f4e20bb.pth) |
 
 **Notes:**
 - All models are trained with 8 GPUs instead of 32 GPUs in the original paper.
 - The warming up lasts for 1 epoch and `2x` here indicates 25 epochs.
-- The training speed is about 3 times slower than Faster R-CNN.