input: 
  from keras.models import Model
  from keras.layers import Input, Dense
  
  def construct_mnist_classifier() -> Model:
      """Construct a neural network model using Keras to classify the MNIST data set.
      The model should have an input layer with shape (784,), a hidden layer with 128 units 
      and ReLU activation, and an output layer with 10 units and softmax activation.
      
      >>> model = construct_mnist_classifier()
      >>> model.summary()
      """
  
output:     input_layer = Input(shape=(784,))
      hidden_layer = Dense(128, activation='relu')(input_layer)
      output_layer = Dense(10, activation='softmax')(hidden_layer)
      model = Model(inputs=input_layer, outputs=output_layer)
      return model