Enables legacy unets to run

[MaxGamill-Sheffield]

Closes #1041

Imports the iou_loss and adds compile=False for non-serialised model objects

Builds on maxgamill-sheffield/unet-crop-fix branch (see #1030) so do this after that

[ns-rse]

Builds on maxgamill-sheffield/unet-crop-fix branch so do this after that

The need to re-add a missing import of mean_iou might be because this branch didn't fork from maxgamill-sheffield/unet-fix-crop rather it forked from main commit 85d65c3 if I've understood the graph history correctly.

But having checked I can't see the mean_iou function being imported on the maxgamill-sheffield/unet-fix-crop branch either so unsure.

If it were meant to build on that earlier branch you should have branched from it and not main (this would have allowed Stacked Pull Requests).

[ns-rse]

Post Code clean, all good to go 👍