make PIL kernels private

[pmeier]

Closes #7808.

cc @vfdev-5

[pytorch-bot]

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[NicolasHug]

LGTM if green but we need to rename to_image_tensor and ToImageTensor as well

[pmeier]

As discussed offline, we are also renaming a few things here:

• ToImageTensor -> ToImage. This also clears up the issue that ToImageTensor returns a datapoints.Image although the name suggests otherwise:

  vision/torchvision/transforms/v2/functional/_type_conversion.py

  Line 20 in e0e6f7e

  return datapoints.Image(output)

• ToImagePIL -> ToPILImage. This is what it was named in v1 and we currently only have it as alias:

  vision/torchvision/transforms/v2/_type_conversion.py

  Lines 80 to 82 in e0e6f7e

  	# We changed the name to align them with the new naming scheme. Still, `ToPILImage` is
  	# prevalent and well understood. Thus, we just alias it without deprecating the old name.
  	ToPILImage = ToImagePIL

  Since the naming scheme is no longer valid, we can also remove the new transform and reinstate the old one.

All arguments here apply to the functional counterparts as well.