Loss function

MOST CHANGES HAPPEN IN THE AIDGE_LEARNING MODULE HERE

Closes #73 (closed)

The choice was made to implement a set of stateless function that can be found in there own directory include/aidge/loss

Exemple of interface:

void MSE(Tensor* input, Tensor* label);

Options like the type of reduction could be added in the future.

void MSE(Tensor* input, Tensor* label, std::string mode = "mean");