It looks like some of your images are RGB (3 channels) images while others might be in grayscale format (1 channel).
Could you try to convert all the RGB using:
item_A = self.transform(Image.open(self.files_A[index % len(self.files_A)]).convert('RGB'))
item_B = self.transform(Image.open(self.files_B[index % len(self.files_B)]).convert('RGB'))

and run it again?