base_model = tf.keras.applications.MobileNetV2(input_shape=[128, 128, 3], include_top=False) # ①

layer_names = [
    'block_1_expand_relu',   # 64x64
    'block_3_expand_relu',   # 32x32
    'block_6_expand_relu',   # 16x16
    'block_13_expand_relu',  # 8x8
    'block_16_project',      # 4x4
] # ②
base_model_outputs = [base_model.get_layer(name).output for name in layer_names] # ③

down_stack = tf.keras.Model(inputs=base_model.input, outputs=base_model_outputs) # ④

down_stack.trainable = False