possibly critical bugs in UNet training script

[till-m]

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In the UNet training script, when training_type="single", I think the model might only be training on one batch element per epoch due to an indentation error. Specifically this if-clause should be indented one more level. The FNO is indented correctly and does not have this problem.

Sidenote (another bug): When constructing the UNetDatasetMult, the script instantiates the class with parameters that don't exist (anymore?), causing a crash.