Add support for decoding 16bits png

[NicolasHug]

This PR adds support for decoding 16bits png in read_image, decode_image and decode_png.

It also adds proper support and tests to convert the resuting torch.uint16 tensor into uint8/float32 tensor via to_dtype().

Note: decoding 16bits pngs was previously supported via a private API that we had to enable to support RAFT. At the time, the torch.uint16 dtype didn't exist, so we had to return a torch.int32 tensor. Support for torch.uint16 was recently added in pytorch. The dtype exists, but very few operators are implemented for it. In our decoder docs, we strongly encourage users to call to_dtype(..., scale=True) to convert the resulting uint16 tensor into a supported dtype like uint8 or float.

Re the "old" v1 F.convert_image_dtype(): uint16 -> float32 works, but -> uint8 fails with a loud error. It's OK, we want users to use the v2 to_dtype anyway.

Closes #4731

[pytorch-bot]

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NEW FAILURES - The following jobs have failed:

• Tests / unittests-windows (3.10, windows.4xlarge, cpu) / windows-job (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 128
• Tests / unittests-windows (3.11, windows.4xlarge, cpu) / windows-job (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 128
• Tests / unittests-windows (3.12, windows.4xlarge, cpu) / windows-job (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 128
• Tests / unittests-windows (3.9, windows.4xlarge, cpu) / windows-job (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 128

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