Emmm. You were changing the wrong part.
Two options for you, you should choose ONE of them depending on your use case:

Change image to three-channel (RGB rather than L mode)

        image = np.array(Image.open(img_path).convert("RGB"))


Change transform to single-channel

train_transform = A.Compose(
        [
            A.Resize(height=IMAGE_HEIGHT, width=IMAGE_WIDTH),
            A.Rotate(limit=35, p=1.0),
            A.HorizontalFlip(p=0.5),
            A.Normalize(
                mean=[0.0],
                std=[1.0],
                max_pixel_value=255.0,
            ),
            ToTensorV2(),
        ]
    )