Add AVIF decoder (Part 1- this is not public or available yet)

[NicolasHug]

This PR adds the ability to decoder AVIF files. It work but I'm keeping it private for now: we have to rely on libavif which is only available on conda-forge, which we cannot use. We'll try to make it available on the pytorch channel.
Until then, building with AVIF support is disabled by default, and _decode_avif is kept private.

Also, we always decode to RGB, e.g. an RGBA image still returns an RGB tensor. The decoder is mostly based on the official example https://github.com/AOMediaCodec/libavif/blob/main/examples/avif_example_decode_memory.c

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/8596

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✅ You can merge normally! (7 Unrelated Failures)

As of commit 640a3d7 with merge base 0a0f34b ():

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

• CMake / windows (windows.4xlarge, cpu) / windows-job (gh) (detected as infra flaky with no log or failing log classifier)
• Tests / unittests-macos (3.9, macos-m1-stable) / macos-job (gh) (detected as infra flaky with no log or failing log classifier)
• Tests / unittests-windows (3.10, windows.4xlarge, cpu) / windows-job (gh) (detected as infra flaky with no log or failing log classifier)
• Tests / unittests-windows (3.11, windows.4xlarge, cpu) / windows-job (gh) (detected as infra flaky with no log or failing log classifier)
• Tests / unittests-windows (3.12, windows.4xlarge, cpu) / windows-job (gh) (detected as infra flaky with no log or failing log classifier)
• Tests / unittests-windows (3.9, windows.4xlarge, cpu) / windows-job (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Tests / unittests-windows (3.9, windows.g5.4xlarge.nvidia.gpu, cuda, 11.8) / windows-job (gh) (trunk failure)
  AttributeError: '_OpNamespace' 'image' object has no attribute '_jpeg_version'

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[github-actions]

Hey @NicolasHug!

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