Working setup with runnable PyTorch on Codex. (#159968)

Sample transcript: https://chatgpt.com/s/cd_68938effc1a88191ae78bc82a8cefe94

This makes use of https://github.com/pytorch/pytorch/pull/159965 to bypass doing an actual build and use nightly.

Things to improve:
- Once USE_NIGHTLY is in main can remove the patching
- We should just keep using the latest nightly, instead of a hard coded one

Signed-off-by: Edward Yang <ezyang@meta.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/159968
Approved by: https://github.com/wdvr

AGENTS.md

@@ -1 +1,17 @@
 - This is the only AGENTS.md, there are no recursive AGENTS.md
+- When you are working on a bug, first create a standalone file that
+  reproduces the bug and verify it fails in the expected way.  Use this to
+  test if your changes work.  Once the change is passing, find an appropriate
+  test file to add the test to and make sure to follow local conventions on
+  the test file.
+- If you are running the real test suite, DO NOT run the entire test suite.
+  Instead run only a single test case, e.g., 'python test/test_torch.py TestTorch.test_dir'
+- Do NOT run setup.py, you do not have a working build environment
+- Do NOT run pre-commit, it is not setup
+- To run lint, run 'lintrunner -a' (which will autoapply changes)
+- Do NOT attempt to install dependencies, you do not have Internet access
+- When you are ready to make a PR, do exactly these steps:
+  - git stash -u
+  - git reset --hard $(cat /tmp/orig_work.txt) # NB: reset to the LOCAL branch, do NOT fetch
+  - git stash pop
+  - Resolve conflicts if necessary