Problem solved. Thanks a million. The transform that solved the problem was this:
transform=transforms.Compose([
                               transforms.Resize(image_size),
                               transforms.CenterCrop(image_size),
                               transforms.ToTensor(),
                               transforms.Lambda(lambda x: x.repeat(3,1,1)),
                               transforms.Normalize((0.5, 0.5, 0.5), (0.5, 0.5, 0.5)),
                               transforms.RandomHorizontalFlip(p=1),
                               transforms.RandomRotation((90,90),expand=False, center=None, fill=0, resample=None),
                               
                              ])