NameError: name '_update_fn' is not defined when using the SAM

Hi,

I installed the ml-backend according to the official steps:
` git clone https://github.com/HumanSignal/label-studio-ml-backend.git`
` cd label-studio-ml-backend/`
` pip install -e .`

Then I installed SAM and started label-studio-ml:
`pip install -r label_studio_ml/examples/segment_anything_model/requirements.txt `
`label-studio-ml start label_studio_ml/examples/segment_anything_model`

It resulted in a success that shows it was running on http://127.0.0.1:8080

But when I started the label studio, and config the SAM model:

I get this error:
label-studio-ml-backend/label_studio_ml/model.py", line 112, in fit
if _update_fn:
NameError: name '_update_fn' is not defined

Could you help with this issue?