[Add] loss function system, MSE loss function, unit-test associated with MSE

Context

First supervised learning features like MSE loss function.

Modified files

• Add loss repository in include/src/unit-tests

assigned to @pineapple

mentioned in merge request aidge_core!78 (merged)

added 1 commit

• 0ec99fd5 - upd SGD

Compare with previous version

mentioned in commit 3ca54187

merged

mentioned in issue aidge_core#81 (closed)