[Fix] Add annotation file of mono3d in demo (#1864)

* add coco ann for mono3d demo and fix inference

* add demo pkl

* update docs

demo/mono_det_demo.py

@@ -19,7 +19,7 @@ def parse_args():
     parser.add_argument(
         '--cam-type',
         type=str,
-        default='CAM_FRONT',
+        default='CAM_BACK',
         help='choose camera type to inference')
     parser.add_argument(
         '--score-thr', type=float, default=0.30, help='bbox score threshold')

docs/zh_cn/user_guides/demo.md

@@ -67,7 +67,7 @@ python demo/mono_det_demo.py ${IMAGE_FILE} ${ANNOTATION_FILE} ${CONFIG_FILE} ${C
 在 nuScenes 数据上测试 [FCOS3D](https://github.com/open-mmlab/mmdetection3d/tree/master/configs/fcos3d) 模型：
 
 ```shell
-python demo/mono_det_demo.py demo/data/nuscenes/n015-2018-07-24-11-22-45+0800__CAM_BACK__1532402927637525.jpg demo/data/nuscenes/n015-2018-07-24-11-22-45+0800__CAM_BACK__1532402927637525_mono3d.coco.json configs/fcos3d/fcos3d_r101-caffe-dcn_fpn_head-gn_8xb2-1x_nus-mono3d_finetune.py checkpoints/fcos3d_r101_caffe_fpn_gn-head_dcn_2x8_1x_nus-mono3d_finetune_20210717_095645-8d806dc2.pth
+python demo/mono_det_demo.py demo/data/nuscenes/n015-2018-07-24-11-22-45+0800__CAM_BACK__1532402927637525.jpg demo/data/nuscenes/n015-2018-07-24-11-22-45+0800__CAM_BACK__1532402927637525_mono3d.pkl configs/fcos3d/fcos3d_r101-caffe-dcn_fpn_head-gn_8xb2-1x_nus-mono3d_finetune.py checkpoints/fcos3d_r101_caffe_fpn_gn-head_dcn_2x8_1x_nus-mono3d_finetune_20210717_095645-8d806dc2.pth
 ```
 
 ### 3D 分割