Added sliding window feature to Giudi Gemma2 model

[slokesha]

Enabled sliding window feature to gemma2 model.
Gemma2 uses sliding window in ever other layer of Gemma2DecoderLayer. The implementation is here . The same logic in GaudiGemma2DecoderLayer.

The value for sliding_window is passed through config file.

Gemma2 can use flash_attention_2 as attention class. Hence enabling flash_attention_2 as a cmdline argument.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

[jiminha]

Do you see perf improvement with this sliding window enabled?