Fixed Softmax impl

Fixed issue with Softmax numerical stability.

This solves aidge_core#150 (closed).

added Fix 🔥🔥 label

assigned to @olivierbichler

enabled an automatic merge when the pipeline for 71e04c1b succeeds

mentioned in issue aidge_core#150 (closed)

mentioned in commit 83b43f32

merged

mentioned in merge request !93 (merged)