Disable download for StanfordCars dataset

[NicolasHug]

URL has been broken for a long time, so we raise a proper error message pointing to #7545 (comment) now.

I couldn't test this change within our dataset test suite because the default mocks would hard-reset the download parameter to False. So hopefully this is an acceptable test:

In [1]: from torchvision.datasets import StanfordCars
In [3]: StanfordCars(root="/tmp", download=True)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-3-a557e888a29e> in <module>
----> 1 StanfordCars(root="/tmp", download=True)

~/dev/vision/torchvision/datasets/stanford_cars.py in __init__(self, root, split, transform, target_transform, download)
     62 
     63         if download:
---> 64             raise ValueError(
     65                 "The original URL is broken so the StanfordCars dataset is not available for automatic "
     66                 "download anymore. You can try to download it manually following "

ValueError: The original URL is broken so the StanfordCars dataset is not available for automatic download anymore. You can try to download it manually following https://github.com/pytorch/vision/issues/7545#issuecomment-1631441616, and set download=False to avoid this error.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/8309

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❌ 2 New Failures

As of commit 7e2e132 with merge base 423a1b0 ():

NEW FAILURES - The following jobs have failed:

• Lint / bc (gh)
  Process completed with exit code 1.
• tests / download (gh)
  Process completed with exit code 4.

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[vfdev-5]

looks good

[NicolasHug]

We could deprecate and remove but it would be disruptive for those who explicitly set download=False already. So I'm tempted to keep it around forever.

[pmeier]

Two minor things, but nothing blocking. Thanks Nicolas!

[NicolasHug]

The tests / download failure is pre-existing and I'm attempting a fix in #8310

The bc job is irrelevant and linux unittests are passing, so this is good to go. Thanks both for the reviews!