fix minmax

[alexriedel1]

📝 Description

This fixed the (critical) bug in the min max scaler.

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[samet-akcay]

ah, thanks for spotting this!

[djdameln]

Ah, this bug seems to have slipped in with my recent fix for multi-GPU support, where I had to make some changes to the post-processing metrics.

Thanks for spotting and fixing it!

On a side note: This should have been caught by the CI, since it impairs the basic functionality of the MinMax metric. It looks like we don't test this metric, so I'll be creating some unit tests for this.