add Imagenette dataset

[pmeier]

Closes #5892. Two non-goals:

• The Imagenette dataset provides "noisy labels" to increase the difficulty. These were not requested in the issue, so I didn't add support for them.
• The "Imagenette dataset family" also provides two similar datasets, but with increased difficulty: "Imagewoof" and "Imagewang". They follow the exact same structure and thus could be easily added by turning the current class into a base class and just parametrizing subclasses. However, again, these datasets were not requested, so I didn't add them.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/8139

• 📄 Preview Python docs built from this PR

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❌ 4 New Failures, 3 Unrelated Failures

As of commit db83c6e with merge base 3feb502 ():

NEW FAILURES - The following jobs have failed:

• Build M1 Conda / pytorch/vision / conda-py3_8-cpu (gh)
• Build M1 Wheels / pytorch/vision / wheel-py3_8-cpu (gh)
• Build Macos Conda / pytorch/vision / conda-py3_8-cpu (gh)
• Build Macos Wheels / pytorch/vision / wheel-py3_8-cpu (gh)

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• Tests / unittests-linux (3.10, linux.12xlarge, cpu) / linux-job (gh)
• Tests / unittests-linux (3.9, linux.12xlarge, cpu) / linux-job (gh)
• Tests / unittests-macos (3.8, macos-m1-12) / macos-job (gh)

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[pmeier]

Lint CI failure is unrelated and fixed in #8140.

[NicolasHug]

Thanks @pmeier , LGTM provided that the doc rendering is OK! Approving now to unblock