Update how loss function work

Context

Currently, Loss function only compute loss but do not create the gradient of the graph output node.

This MR aims at fixing this issue following the solution described in: #1 (closed)

Modified files

• MSE function now set the mGrad of the pred Tensor.
• Bind loss function MSE

.gitlab/ci/build.gitlab-ci.yml

@@ -15,7 +15,6 @@ build:ubuntu_cpp:
     # aidge_backend_cpu
     - DEPENDENCY_NAME="aidge_backend_cpu"
     - !reference [.download_dependency, script]
-
     # Build current module
     - export CMAKE_PREFIX_PATH=../install_cpp
     - mkdir -p build_cpp