Fix dead LeNet/MNIST download for fgsm_tutorial

The Google Drive link for the fgsm_tutorial LeNet/MNIST checkpoint started
returning HTTP 404, which broke `make download` on any build shard that runs
fgsm_tutorial. Point the download at a public S3 mirror
(s3://pytorch-tutorial-assets/lenet_mnist_model_v2.pth) instead.

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

Author: yushangdi

.jenkins/download_data.py

@@ -99,10 +99,10 @@ def download_dcgan_data() -> None:
 
 def download_lenet_mnist() -> None:
     # Download model for beginner_source/fgsm_tutorial.py
-    download_url_to_file("https://docs.google.com/uc?export=download&id=1HJV2nUHJqclXQ8flKvcWmjZ-OU5DGatl",
+    download_url_to_file("https://s3.amazonaws.com/pytorch-tutorial-assets/lenet_mnist_model_v2.pth",
                          prefix=BEGINNER_DATA_DIR,
                          dst="lenet_mnist_model.pth",
-                         sha256="cb5f8e578aef96d5c1a2cc5695e1aa9bbf4d0fe00d25760eeebaaac6ebc2edcb",
+                         sha256="c042f14781771cb6c58c833e9752fe89544d12c0649d0f2fc53f00a4ef09229c",
                          )
 
 def download_gpu_quantization_torchao() -> None: