input: Write a function to construct a neural network model in Python to classify the MNIST data set correctly. 
  
  assert construct_mnist_model()==keras.Model(inputs=keras.layers.Input(shape=(784,)), outputs=keras.layers.Dense(10, activation='softmax')(keras.layers.Dense(128, activation='relu')(keras.layers.Input(shape=(784,)))))
output: def construct_mnist_model():
      inputs = keras.layers.Input(shape=(784,))
      x = keras.layers.Dense(128, activation='relu')(inputs)
      outputs = keras.layers.Dense(10, activation='softmax')(x)
      return keras.Model(inputs=inputs, outputs=outputs)