added some more details + debugging steps to CONTRIBUTING.md (#45903)

Summary:
When attempting to install PyTorch locally on my Macbook, I had some difficulty running the setup steps and understanding what I was really doing. I've added some clarifications and summarized some debugging steps about `python setup.py develop` to lower the barrier of entrance for new contributors.

I'm seeking a lot of review here since I am not sure if what I wrote is entirely the most useful or accurate. Thank you!

Pull Request resolved: https://github.com/pytorch/pytorch/pull/45903

Reviewed By: albanD

Differential Revision: D24140343

Pulled By: janeyx99

fbshipit-source-id: a5e40d1bc804945ae7db2b95ab18cf7fe169e68a

1 files changed, 27 insertions, 1 deletions

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index b011846039..a1b4096592 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -118,11 +118,37 @@ For example:
 - modify your Python file `torch/__init__.py`
 - test functionality
 
-You do not need to repeatedly install after modifying Python files.
+You do not need to repeatedly install after modifying Python files (`.py`). However, you would need to reinstall
+if you modify Python interface (`.pyi`, `.pyi.in`) or non-Python files (`.cpp`, `.cc`, `.cu`, `.h`, ...).
 
 In case you want to reinstall, make sure that you uninstall PyTorch first by running `pip uninstall torch`
 and `python setup.py clean`. Then you can install in `develop` mode again.
 
+### Tips and Debugging
+* A prerequisite to installing PyTorch is CMake. We recommend installing it with [Homebrew](https://brew.sh/)
+with `brew install cmake` if you are developing on MacOS or Linux system.
+* Our `setup.py` requires Python >= 3.6
+* If you run into errors when running `python setup.py develop`, here are some debugging steps:
+  1. Run `printf '#include <stdio.h>\nint main() { printf("Hello World");}'|clang -x c -; ./a.out` to make sure
+  your CMake works and can compile this simple Hello World program without errors.
+  2. Nuke your `build` directory. The `setup.py` script compiles binaries into the `build` folder and caches many
+  details along the way, which saves time the next time you build. If you're running into issues, you can always
+  `rm -rf build` from the toplevel `pytorch` directory and start over.
+  3. If you have made edits to the PyTorch repo, commit any change you'd like to keep and clean the repo with the
+  following commands (note that clean _really_ removes all untracked files and changes.):
+  ```bash
+  git submodule deinit -f .
+  git clean -xdf
+  python setup.py clean
+  git submodule update --init --recursive # very important to sync the submodules
+  python setup.py develop                 # then try running the command again
+  ```
+  4. The main step within `python setup.py develop` is running `make` from the `build` directory. If you want to
+  experiment with some environment variables, you can pass them into the command:
+  ```bash
+  ENV_KEY1=ENV_VAL1[, ENV_KEY2=ENV_VAL2]* python setup.py develop
+  ```
+
 ## Nightly Checkout & Pull
 
 The `tools/nightly.py` script is provided to ease pure Python development of