RGB<->Grayscale CV-CUDA Backend

[justincdavis]

Summary

Implements the rgb_to_grayscale and grayscale_to_rgb kernels for CV-CUDA tensors.

[pytorch-bot]

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❌ 5 New Failures

As of commit f5459e3 with merge base aa35ca1 ():

NEW FAILURES - The following jobs have failed:

• Tests / unittests-linux (3.11, linux.12xlarge, cpu) / linux-job (gh)
  test/test_datasets.py::FlyingChairsTestCase::test_transforms
• Tests / unittests-linux (3.12, linux.12xlarge, cpu) / linux-job (gh)
  test/test_datasets.py::FlyingChairsTestCase::test_transforms
• Tests / unittests-macos (3.12, macos-m1-stable) / macos-job (gh)
  test/test_datasets.py::FlyingChairsTestCase::test_transforms
• Tests / unittests-windows (3.11, windows.4xlarge, cpu) / windows-job (gh)
  test/test_datasets.py::FlyingChairsTestCase::test_transforms
• Tests / unittests-windows (3.12, windows.4xlarge, cpu) / windows-job (gh)
  test/test_datasets.py::FlyingChairsTestCase::test_transforms

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[zy1git]

Not part of this PR, but for consistency I'd suggest:
Use assert_equal(a, b) when exact equality is required
Use assert_close(a, b, rtol=..., atol=...) when tolerance is needed
Using assert_equal(..., atol=1) works but is semantically confusing since 'equal' implies exact match.
@NicolasHug Do you think we need to address this?

[zy1git]

This line might be written in if num_output_channels not in (1, 3): to be consistent with the function _rgb_to_grayscale_image_pil above.

[zy1git]

We don't need to reassign gray, instead, we can do return cvcuda.cvtcolor(gray, cvcuda.ColorConversion.GRAY2RGB) as above (line 89).