I am on a linux machine so the way I do it might be different from you. Nevertheless, one way to add the torch_lib_dir on linux is as follows

If you are in the build directory (one down from the cmake file), type ccmake .. which open the cmake gui.
Then, you will see a line stating: TORCH_DIR. You can set this to the path containing libtorch/share/cmake/Torch.  In my case this is home/fabian/torch_cpp/libtorch/share/cmake/Torch

Configure and Generate the cmake file (press c and g)

With these steps I could resolve the error torch_dir not found. Do these steps work for you?