make load_and_preprocess_images accept grayscale image

[HyeonJaeGil]

Current load_and_preprocess_images function only takes RGB images,
but some domains (ex. thermal images) are working with grayscale images.

The purpose of this PR is to add an option to choose between RGB / Grayscale for user's input.

[jytime]

Hi,

Since our model expects 3-channel input images, I believe that even grayscale inputs would need to be converted to a 3-channel format (e.g., by converting to RGB or duplicating the single channel).
Using img = img.convert("L") produces a single-channel image, which seems incompatible with our model's requirements.
Please let me know if this is correct, or if I've misunderstood something.