Don't print when dataset is already downloaded

[NicolasHug]

Closes #8680

Also made a change to ImageNette so that download=True doesn't raise if the dataset is already there., as requested in #8638

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/8681

• 📄 Preview Python docs built from this PR

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

❌ 3 New Failures

As of commit fe0ac3a with merge base a7f0d25 ():

NEW FAILURES - The following jobs have failed:

• Lint / c-source / linux-job (gh)
  RuntimeError: Command docker exec -t 3ee493a2a4457572e7a6abf81f64144e6e935631307682b4eab8762d9cc84d9a /exec failed with exit code 1
• Tests / unittests-windows (3.10, windows.4xlarge, cpu) / windows-job (gh)
  ##[error]fatal: couldn't find remote ref refs/pull/8681/merge
• Tests / unittests-windows (3.9, windows.g5.4xlarge.nvidia.gpu, cuda, 11.8) / windows-job (gh)
  ##[error]fatal: couldn't find remote ref refs/pull/8681/merge

This comment was automatically generated by Dr. CI and updates every 15 minutes.