When building with Setuptools, the tool creates a build directory in the distribution root, i.e. the same location as the setup.py file. Because PyTorch is composed of both "Pure" python modules and Extension Modules, we need to preserve information about the Operating System and Python version used when performing the build. So if we look in my build directory, we see:
(p3) killeent@devgpu047:pytorch (master)$ ls build
lib.linux-x86_64-3.6  temp.linux-x86_64-3.6

This indicates that I've built the project on linux-x86-64 using Python 3.6. The lib directory contains the library files, while the temp directory contains files generated during the build that aren't needed in the final installation.
Because "Pure" python modules are just Python code, and don't need to be "compiled", the build_py process simply copies files from their locations as found by find_packages to the equivalent location in build/. So our build output is littered with lines like:
```bash