input: Write a function to generate a neural network model for XOR problem using TensorFlow.
  assert isinstance(generate_xor_nn(), tf.keras.Model)
output: import tensorflow as tf
  
  def generate_xor_nn():
      model = tf.keras.Sequential([
          tf.keras.layers.Dense(2, input_shape=(2,), activation='relu'),
          tf.keras.layers.Dense(1, activation='sigmoid')
      ])
      model.compile(optimizer='adam', loss='binary_crossentropy', metrics=['accuracy'])
      return model