Is your feature request related to a problem? Please describe.
WSIReader is intended to be used for "Whole Slide Images" as its name shows and not to load arbitrary tiff files.
When a tiff file without the color channel is being loaded using WSIReader, currently the different backends handles is differently. Refer to #3858

To have a unified interface, all those backends should raise an error in this situations. For different backends:

• OpenSlide: already throw an error
• CuCIM: repeat the values to create an RGBA image. An issue is open to fix this: Images with no color channel! rapidsai/cucim#230
• Tiffile: it load a 2D array. This needs to be addressed in WSIReader.