fix: adjust warning

test_kernel.py

@@ -25,5 +25,5 @@ gray_tensor = img2gray_lib.img2gray(img_tensor).squeeze()
 print(gray_tensor.shape)  # HW
 
 # save the output image
-gray_img = Image.fromarray(gray_tensor.cpu().numpy().astype(np.uint8), mode="L")
+gray_img = Image.fromarray(gray_tensor.cpu().numpy().astype(np.uint8))
 gray_img.save("kernel-builder-logo-gray2.png")