Add support for Gemma models

[lullabies777]

Gemma 7B: https://huggingface.co/google/gemma-7b Gemma 2B: https://huggingface.co/google/gemma-2b Blog: https://blog.google/technology/developers/gemma-open-models/ Paper: https://storage.googleapis.com/deepmind-media/gemma/gemma-report.pdf

[chqiwang]

+1