Add BCE loss function

Context

Add Binary Cross Entropy (BCE) loss function for classification.

Detailed major modifications

BCE loss function is computed using a graph in order to not be backend dependant.

A unit test is added.

include/aidge/loss/LossList.hpp

@@ -31,6 +31,8 @@ namespace loss {
  */
 Tensor MSE(std::shared_ptr<Tensor>& prediction,
            const std::shared_ptr<Tensor>& target);
+Tensor BCE(std::shared_ptr<Tensor>& prediction,
+           const std::shared_ptr<Tensor>& target);
 
 }  // namespace loss
 }  // namespace Aidge