If you’ve installed the PyTorch binaries via conda, you should fine some files in your conda directory, e.g.:
/opt/conda/lib/python3.6/site-packages/torch/include/...

However, modifying them won’t have any effect.
Instead, you should build PyTorch from source as described here 3.
Also, the Contributing guidelines 1 might be helpful.
If you change some files in the source folder and rebuild, you will be able to see the changes.