Ranker tag not displayed

**Describe the bug**
When I following the instruction in https://labelstud.io/tags/ranker.html to create a ranker tag, I found it not be displayed in the interface.

**To Reproduce**
Steps to reproduce the behavior:
1.Save the example JSON below as a file called example_ranker_tag.json.
2.Upload it as a task on the Label Studio UI.
3.Set up a project with the given labeling configuration.

**Expected behavior**
Show Ranker tag normally for the RLHF annotation

**Screenshots**

**Environment (please complete the following information):**
- Ubuntu 18.04.6
- Label Studio Version 1.7.1

**Additional context**
Really important for building a RLHF annotation platform