请问ctc_2019中input_length=16, label_length=4的具体使用方法？ about captcha_break HOT 8 OPEN

from tensorflow.keras.utils import Sequence

class CaptchaSequence(Sequence):
    def __init__(self, characters, batch_size, steps, n_len=4, width=128, height=64, 
                 input_length=16, label_length=4):
        self.characters = characters
        self.batch_size = batch_size
        self.steps = steps
        self.n_len = n_len
        self.width = width
        self.height = height
        self.input_length = input_length
        self.label_length = label_length
        self.n_class = len(characters)
        self.generator = ImageCaptcha(width=width, height=height)
    
    def __len__(self):
        return self.steps

    def __getitem__(self, idx):
        X = np.zeros((self.batch_size, self.height, self.width, 3), dtype=np.float32)
        y = np.zeros((self.batch_size, self.n_len), dtype=np.uint8)
        input_length = np.ones(self.batch_size)*self.input_length
        label_length = np.ones(self.batch_size)*self.label_length
        for i in range(self.batch_size):
            random_str = ''.join([random.choice(self.characters) for j in range(self.n_len)])
            X[i] = np.array(self.generator.generate_image(random_str)) / 255.0
            y[i] = [self.characters.find(x) for x in random_str]
        return [X, y, input_length, label_length], np.ones(self.batch_size)

import tensorflow.keras.backend as K

def ctc_lambda_func(args):
    y_pred, labels, input_length, label_length = args
    return K.ctc_batch_cost(labels, y_pred, input_length, label_length)