AttributeError when opening ListModelVersions in the browser with a model ID that does not exist

Sentry Issue: ARKINDEX-BACKEND-1SN

AttributeError: 'ModelVersionsList' object has no attribute 'model'
(4 additional frame(s) were not displayed)
...
  File "arkindex/training/api.py", line 109, in get_serializer_context

This has a similar root cause to #1458 (closed). The model is not actually needed in the serializer context, we could use the model_id alone, and it should be made a cached property anyway.