Fix png palette decoding when mode is UNCHANGED.

[NicolasHug]

A palette png with mode=UNCHANGED was returning raw palette indices - that's incorrect. This PR fixes it, aligning the behavior with PIL's.

Oh, this also adds num_threads to avif - oops, forgot to switch branches. Oh well.

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/meta-pytorch/torchcodec/1543

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• Unrelated MacOS MPS test jobs time out (30 min) due to stale cpp_extension lock file poisoning self-hosted runners

❌ 1 New Failure, 10 Pending

As of commit 1058cac with merge base e732df2 ():

NEW FAILURE - The following job has failed:

• Linux CUDA aarch64 / install-and-test (3.10, 12.6, 7) (gh)
  Process completed with exit code 2.

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