Update cmake to set faust_torch module available by FAUST_TORCH cmake option...

Update cmake to set faust_torch module available by FAUST_TORCH cmake option both in C++ tests and pyfaust (if of course libtorch is installed).

cmake -DCMAKE_PREFIX_PATH=/my/path/to/libtorch  -DFAUST_TORCH=ON -DBUILD_WRAPPER_PYTHON=ON ..