Serialized model changes input nodes orders

[haifeng-jin]

Another issue from AdBrain about reconstructing the model but the layer order changed in the serialized model.

Colab: https://colab.corp.google.com/drive/1tSZqhkpLjrfPgmpQkkbLC4Ej41CuFokR?resourcekey=0-oicPFmdbi-P_jVx4C0XqRw&usp=sharing