Cortex-M: preserve calibration input layout

[rascani]

Summary

Calibration observers use logical tensor values, so avoid converting calibration samples to channels-last. Cover the behavior in the MobileNet tests and clarify the documentation.

Authored with Codex.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils @Sebastian-Larsson @robell

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