I received this email from pascal (see above) :

Hello Mathieu,
In your cmakelists.txt, add
set(CMAKE_PREFIX_PATH  ${Torch_DIR}) 

or (which I find not more convenient)
cd build
cmake -DCMAKE_PREFIX_PATH=/home/mathieu/Thales/example-app/libtorch ..

cmake is not looking for the .so file, refer to cmake find_package for more info.

I did what he asked me and it worked