Softmax Backward implementation for cpu

Context

This MR introduces a general implementation of the softmax backward function, as outlined in the related issue aidge#251.

Modified files

• include/aidge/backend/cpu/operator/SoftmaxImpl.hpp, include/aidge/backend/cpu/operator/SoftmaxImpl_kernels.hpp , src/operator/SoftmaxImpl.cpp : add backward implementation for the operator
• unit_tests/operator/Test_SoftmaxImpl.cpp : Add unit tests for softmax backward

Detailed major modifications

This MR follows up on !150. The implementation has been updated to use a general derivative formula and to address the feedback in !150 (comment 3650965). Additionally, the unit tests have been revised based on the feedback in comment !150 (comment 3393682) and the new kernel implementation. Looking forward to your feedback 😄