Allow decode_image to support paths

[NicolasHug]

I assume we needed both decode_image and read_image back then when torchscript didn't support Union. But now that it does, it doesn't make much sense to have both.

This PR allows decode_image to support paths (as strings and Pathlib.Path), on top of tensors. read_image is now marked as obsolete. I'm not deprecating it, it's not worth the effort.

[pytorch-bot]

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✅ You can merge normally! (3 Unrelated Failures)

As of commit 0aa8a31 with merge base c36025a ():

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

• Build M1 Conda / pytorch/vision / upload / conda-py3_9-cpu (gh) (detected as infra flaky with no log or failing log classifier)
• Lint / bc (gh) (similar failure)
• Tests / unittests-windows (3.9, windows.g5.4xlarge.nvidia.gpu, cuda, 11.8) / windows-job (gh) (detected as infra flaky with no log or failing log classifier)

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