AttributeError: 'NoneType' object has no attribute 'prepare'

Hello,

I got an error in insightface package of retinaFace.

```
import insightface
model = insightface.model_zoo.get_model('retinaface_r50_v1')
model.prepare(ctx_id=-1, nms=0.4)
```

error:

AttributeError: 'NoneType' object has no attribute 'prepare'

Reproduce the provided Python snippet and inspect the insightface.model_zoo.get_model entry point to determine why it returns None for retinaface_r50_v1. Trace the model lookup and loading path, then verify that the call returns a usable model whose prepare method runs without this AttributeError.

Written by the indexing model from the issue text.