Different results for same image during load_image

[sizov-kirill]

The function load_image can return different images for different backends.
This issue was discovered while experimenting with loading class masks from the original Mapillary Vistas dataset.
If the function load_image using PIL backend for reading mapillary mask, then it returns array with the shape [H, W, 1],
at the same time if load_image using OpenCV backend, then it returns array with the shape [H, W, 3].

Such behavior could potentially affects reading datasets in other formats too.

[zhiltsov-max]

Consider adding / extending tests for different backends. Maybe, running all tests with non-default backed is a good idea.