can not display pseduo polygon labels on the web pic

**Describe the bug**
We are using label studio to use Openmmlab maskrcnn polygon labels output. But we found there is no output on the web.
**To Reproduce**
Steps to reproduce the behavior:
1.My backend code and config file is in the attachment.
**Expected behavior**
I expect to see the openmmlab labeled labels displayed on the webpage.
**Screenshots**

**Environment (please complete the following information):**
- OS: ubuntu
- Label Studio Version:1.7.3
- openmmlab version: mmcv_full 1.6.0 mmdet 2.25.0
- label-studio-ml :1.0.8rc3

**Additional context**
Add any other context about the problem here.
[ml_bkend_maskrcnn_polygon.zip](https://github.com/heartexlabs/label-studio/files/11572782/ml_bkend_maskrcnn_polygon.zip)