One Channel Input Image in ImageDataLoaders.from_df missing x_block

When I try to use a single channel image as input with appropriate mapping in dataframe, I thought ImageDataLoader.from_df was a convenient function to do so. But there wasn’t any provision to pass a one channel block. I created a method to do the same for my purpose. Though adding a parameter for x_block might have been better. I could do a PR for the same. Just wanted to know if there is a simpler option.