PyTorch PR maps 9 CMake build stages to help developers navigate the build system

PyTorch's build system has long been a maze: the root CMakeLists.txt is thousands of lines, and finding where a specific build step lives meant reading the entire file. PR #193615, authored by albanD with the diff generated and reviewed by Claude Code, fixes this with a comment-only change that adds a nine-stage map at the top of the file. The stages are listed in execution order: bootstrap and env-var forwarding, pre-build submodule/NCCL setup, options, dependencies, codegen, the main build, export plus optional binaries/JNI, Python packaging, and the configuration summary. Each stage names the specific cmake/ module or subdirectory that implements it, giving developers direct pointers instead of guesswork.

The PR also calls out the environment variables that configure the build, pointing Stage 1 at cmake/EnvVarForwarding.cmake, while noting exceptions like third-party paths (MKLROOT, ROCM_PATH) and a few niche flags read directly where they are used. Both the spin develop and spin install docstrings gain a sentence referencing the new overview and the module list. Each stage description was cross-checked against its named module, and the change was verified with BUILD_CONFIG spin develop, spin develop --help, spin install --help, and spin lint. As a comment-only PR, it introduces zero risk to build behavior—just a significant improvement in developer experience for anyone touching PyTorch's build configuration.