Add Gemma 3

[jackzhxng]

🚀 The feature, motivation and pitch

Add support for the text decoder backbone of the new Gemma 3 (1B / 4B for edge) model.

Use --local_global_attention for the sliding window attention.

Checkpoints are on HuggingFace:

• google/gemma-3-4b-it
• google/gemma-3-1b-it

cc @mergennachin @iseeyuan @lucylq @helunwencser @tarun292 @kimishpatel