ADD mobilebert

https://dev.classmethod.jp/articles/mobilebert-with-python/

Start with the linked MobileBERT article and inspect how existing model additions are organized in ailia-models. Determine the required model files, entry point, and validation steps before proceeding; done should mean MobileBERT is added consistently and can be run successfully.

索引モデルが issue の本文から書いたものです。