Fix dead LeNet/MNIST download for fgsm_tutorial

[yushangdi]

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

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/tutorials/3922

• 📄 Preview Python docs built from this PR

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As of commit f7f128f with merge base 326edb0 ():
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