Improve ImageNet documentation

[puhuk]

Update to download before using the ImageNet class.

cc @pmeier

[pytorch-bot]

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❌ 25 Failures

As of commit 531fd0b:

NEW FAILURES - The following jobs have failed:

• build / linux-job (gh)
• c-source / linux-job (gh)
• onnx / linux-job (gh)
• python-source-and-configs / linux-job (gh)
• python-types / linux-job (gh)
• unittests (3.10, linux.12xlarge, cpu) / linux-job (gh)
• unittests (3.10, macos-12) / macos-job (gh)
• unittests (3.10, windows.4xlarge, cpu) / windows-job (gh)
• unittests (3.11, linux.12xlarge, cpu) / linux-job (gh)
• unittests (3.11, macos-12) / macos-job (gh)
• unittests (3.11, windows.4xlarge, cpu) / windows-job (gh)
• unittests (3.8, linux.12xlarge, cpu) / linux-job (gh)
• unittests (3.8, linux.g5.4xlarge.nvidia.gpu, cuda, 11.7) / linux-job (gh)
• unittests (3.8, macos-12) / macos-job (gh)
• unittests (3.8, macos-m1-12) / macos-job (gh)
• unittests (3.8, windows.4xlarge, cpu) / windows-job (gh)
• unittests (3.9, linux.12xlarge, cpu) / linux-job (gh)
• unittests (3.9, macos-12) / macos-job (gh)
• unittests (3.9, windows.4xlarge, cpu) / windows-job (gh)
• unittests-extended / linux-job (gh)
• unittests-prototype (3.10, linux.12xlarge, cpu) / linux-job (gh)
• unittests-prototype (3.11, linux.12xlarge, cpu) / linux-job (gh)
• unittests-prototype (3.8, linux.12xlarge, cpu) / linux-job (gh)
• unittests-prototype (3.8, linux.g5.4xlarge.nvidia.gpu, cuda, 11.7) / linux-job (gh)
• unittests-prototype (3.9, linux.12xlarge, cpu) / linux-job (gh)

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[pmeier]

Thanks @puhuk! One minor nit inline. Otherwise LGTM!

Rendered docs

[NicolasHug]

Thanks @puhuk