Support grayscale images in numpy_to_pil

[anton-l]

This fixes the error when trying to convert single-channel arrays to PIL

images = np.random.rand(2, 32, 32, 1)
numpy_to_pil(images)  

Before:

TypeError: Cannot handle this data type: (1, 1, 1), |u1

After:

[<PIL.Image.Image image mode=L size=32x32 at 0x7FCF8F8FE690>,
<PIL.Image.Image image mode=L size=32x32 at 0x7FCF8F8FE6D0>]

Closes #488

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[pcuenca]

Does this ever come up during regular pipeline use?

[anton-l]

@pcuenca not in our usual cases, but was brought up in #488

[pcuenca]

Oh, it makes total sense for training, of course.