Support training continuation (pretrained models) for teacher ensembles

[gregtatum]

The training_continuation.py doesn't correctly handle multiple URLs.

Steps to reproduce:

Set the config

experiment:
  pretrained-models:
    train-teacher:
      urls:
        - https://example.com/a
        - https://example.com/b
      mode: use
      type: default

Run task preflight-check

Open artifacts/full-task-graph.json.

Search for both example.com/a and example.com/b in the generated tasks. train-teacher-XX-XX-1/2 and train-teacher-XX-XX-2/2 both contain the same files, and the URLs aren't split between them.