nnunet v2: continue training with pretrained weights

In order to upgrade the DAG dag_train_with_pretrained_weights.py to nnU-Net V2, we need to update the functionality in the following way:

• adopt this functionality: https://github.com/MIC-DKFZ/nnUNet/blob/master/documentation/pretraining_and_finetuning.md
• Note: nnU-Net V1 had specialized warm-up trainers that are not given anymore in V2, therefore we simply follow their fine-tuning from pre-training suggestion from above