input: Write a function to construct a neural network model in Python using Keras to classify the MNIST data set correctly. The function should return the compiled model.
  assert type(construct_mnist_model())==keras.Model
  
output: def construct_mnist_model():
      from tensorflow.keras import Sequential
      from tensorflow.keras.layers import Dense, Flatten
      model = Sequential([
          Flatten(input_shape=(28, 28)),
          Dense(128, activation='relu'),
          Dense(10, activation='softmax')
      ])
      model.compile(optimizer='adam', loss='sparse_categorical_crossentropy', metrics=['accuracy'])
      return model