## Summary

`make download` fails on any build shard that runs `fgsm_tutorial`
because the LeNet/MNIST checkpoint it depends on was fetched from a
Google Drive link that now returns **HTTP 404**:

```
.jenkins/download_data.py -> download_lenet_mnist()
  https://docs.google.com/uc?export=download&id=1HJV2nUHJqclXQ8flKvcWmjZ-OU5DGatl
  -> urllib.error.HTTPError: HTTP Error 404: Not Found
make: *** [Makefile:73: docs] Error 2
```

This is a pre-existing, repo-wide breakage (the link died after the last
`main` build, so it only surfaces on PRs whose shard rebalancing puts
`fgsm_tutorial` on a worker that exercises the download).

## Fix

Point `download_lenet_mnist()` at a public S3 mirror on
`pytorch-tutorial-assets` (the same host the DCGAN download already
uses) instead of the dead Drive link.

The checkpoint was retrained to match the exact `Net` architecture in
`beginner_source/fgsm_tutorial.py` (conv1 32x1x3x3 / conv2 / fc1
9216x128 / fc2), so it loads with a strict `load_state_dict`.

## Verification

- Anonymous public fetch of the new S3 URL returns HTTP 200 with the
pinned sha256.
- Ran `download_lenet_mnist()` end-to-end: file lands at
`beginner_source/data/lenet_mnist_model.pth`, sha256 matches.
- Confirmed the checkpoint loads strictly into the current tutorial
`Net` and runs an FGSM step.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>