原代码：
X = []
for p in tqdm(train_fns):
img=cv2.imread(TRAIN_IMAGE_DIR+p,cv2.IMREAD_GRAYSCALE)
img = np.array(img,dtype=np.uint8)
X.append(img)
优化后的代码：
X =[np.array(cv2.imread(TRAIN_IMAGE_DIR+p,cv2.IMREAD_GRAYSCALE),dtype=np.uint8) for p in tqdm(train_fns)]
