remove invalid mnist download links

[csukuangfj]

The following links are no longer working.

• https://yann.lecun.com/exdb/mnist/train-images-idx3-ubyte.gz
• https://yann.lecun.com/exdb/mnist/train-labels-idx1-ubyte.gz
• https://yann.lecun.com/exdb/mnist/t10k-images-idx3-ubyte.gz
• https://yann.lecun.com/exdb/mnist/t10k-labels-idx1-ubyte.gz

See also the screenshot below.

Removing the invalid link gives us a cleaner log.

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/8572

• :page_facing_up: Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

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[NicolasHug]

Thanks for the PR @csukuangfj . The first original mirror tends to work on and off. I agree the log isn't optimal when we fallback to the second mirror, but because that second mirror tends to fail sometimes as well, it's better to keep the first one. Hopefully it will be back live soon.