This should help -> PyTorch 1.3 wheels for Raspberry Pi (Python 3.7)
You’ll find the dynamic library in something like /home/pi/.local/lib/python3.7/site-packages/torch/lib, and the header files in /home/pi/.local/lib/python3.7/site-packages/torch/include/torch/csrc/api/include or / and /home/pi/.local/lib/python3.7/site-packages/torch/include.