input: Write a python function to preprocess a list of sentences using Keras Tokenizer and pad the sequences.
  assert (preprocess_sentences(['This is great', 'I am happy'], maxlen=5) == [[1, 2, 3, 0, 0], [4, 5, 6, 0, 0]]).all()
  assert (preprocess_sentences(['We love coding', 'Python is fun'], maxlen=4) == [[1, 2, 3, 0], [4, 5, 6, 0]]).all()
  assert (preprocess_sentences(['TensorFlow is powerful', 'Keras makes it easy'], maxlen=6) == [[1, 2, 3, 4, 0, 0], [5, 6, 7, 8, 0, 0]]).all()
output: def preprocess_sentences(sentences, maxlen):
      from keras.preprocessing.text import Tokenizer
      from keras.preprocessing.sequence import pad_sequences
      import numpy as np
  
      tokenizer = Tokenizer()
      tokenizer.fit_on_texts(sentences)
      sequences = tokenizer.texts_to_sequences(sentences)
      padded_sequences = pad_sequences(sequences, maxlen=maxlen)
      
      return np.array(padded_sequences)