Draft: [Issue #251] : WIP: Softmax Backward implementation for cpu

Context

Through this Merge-Request is illustrated the code for the backward cpu implementation of Softmax operator (find related issue here).

The current code is only preliminary, it is likely not mature enough. Thus, the purpose of this MR is only to get feedback for further development. In addition, some implementation choice have been made through the issue, choice that might not be appropriate. Please validate or invalidate them.

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 (once the implementation validated, additional tests cases will be added)

TODO

• Backward Implementation validated
• Add other test cases