perf(Gemma4): reduce comms in sliding window, FA implementation, (#2984)

* Gemma4: reduce comms in sliding window, FA implementation,

Signed-off-by: Nathan Azrak <nathan.azrak@gmail.com>

* testing suite

Signed-off-by: Nathan Azrak <nathan.azrak@gmail.com>

* fix(gemma4): preserve vision CP attention semantics

Signed-off-by: HuiyingLi <willwin.lee@gmail.com>

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Signed-off-by: Nathan Azrak <nathan.azrak@gmail.com>
Signed-off-by: HuiyingLi <willwin.lee@gmail.com>
Co-authored-by: Alexandros Koumparoulis <153118171+akoumpa@users.noreply.github.com>
Co-authored-by: HuiyingLi <willwin.lee@gmail.com>