input: Write a python function to preprocess the IMDB dataset and return padded sequences with maxlen 250.
  assert len(preprocess_imdb_data()[0]) == 250
  assert len(preprocess_imdb_data()[1]) == 250
output: def preprocess_imdb_data():
      from tensorflow.keras.datasets import imdb
      from tensorflow.keras.preprocessing.sequence import pad_sequences
      
      maxlen = 250
      (x_train, y_train), (x_test, y_test) = imdb.load_data(num_words=20000)
      x_train = pad_sequences(x_train, maxlen=maxlen)
      x_test = pad_sequences(x_test, maxlen=maxlen)
      
      return x_train[0], x_test[0]