Add fp16 Retinanet benchmark (#3251)

* add fp16 benchmark

* fix typo

configs/fp16/README.md

@@ -16,3 +16,4 @@
 |:------------:|:---------:|:-------:|:-------:|:--------:|:--------------:|:------:|:-------:|:--------:|
 | Faster R-CNN | R-50      | pytorch | 1x      | 3.4      | 28.8           | 37.5   | -       |[model](https://open-mmlab.s3.ap-northeast-2.amazonaws.com/mmdetection/v2.0/fp16/faster_rcnn_r50_fpn_fp16_1x_coco/faster_rcnn_r50_fpn_fp16_1x_coco_20200204-d4dc1471.pth) | [log](https://open-mmlab.s3.ap-northeast-2.amazonaws.com/mmdetection/v2.0/fp16/faster_rcnn_r50_fpn_fp16_1x_coco/faster_rcnn_r50_fpn_fp16_1x_coco_20200204_143530.log.json) |
 | Mask   R-CNN | R-50      | pytorch | 1x      | 3.6      | 24.1           | 38.1   | 34.7    |[model](https://open-mmlab.s3.ap-northeast-2.amazonaws.com/mmdetection/v2.0/fp16/mask_rcnn_r50_fpn_fp16_1x_coco/mask_rcnn_r50_fpn_fp16_1x_coco_20200205-59faf7e4.pth) | [log](https://open-mmlab.s3.ap-northeast-2.amazonaws.com/mmdetection/v2.0/fp16/mask_rcnn_r50_fpn_fp16_1x_coco/mask_rcnn_r50_fpn_fp16_1x_coco_20200205_130539.log.json) |
+| Retinanet    | R-50      | pytorch | 1x      | 2.8      | 31.6           | 36.4  |     |[model](https://open-mmlab.s3.ap-northeast-2.amazonaws.com/mmdetection/v2.0/fp16/retinanet_r50_fpn_fp16_1x_coco/retinanet_r50_fpn_fp16_1x_coco_20200702-0dbfb212.pth) | [log](https://open-mmlab.s3.ap-northeast-2.amazonaws.com/mmdetection/v2.0/fp16/retinanet_r50_fpn_fp16_1x_coco/retinanet_r50_fpn_fp16_1x_coco_20200702_020127.log.json) |