Support fine-tuning

TODO:

load model version files
move weights.ckpt to self.workdir / "experiment" / "<model_version_id>.ckpt"
set common.checkpoint = "<model_version_id>.ckpt" https://gitlab.teklia.com/workers/pylaia/-/blob/8d0ec5787161307f80a3d65e295edfcaa0690e93/worker_pylaia/train.py#L138
use the value of max_epochs in train.resume

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