Laguna: support per-element output gating (#46690)
* Laguna: support per-element output gating
Dispatch on config.gating: True/"per-head" keeps one gate per head
(g_proj=num_heads), "per-element" sizes g_proj at num_heads*head_dim.
Regenerate modeling/configuration and add gating shape tests.
* Address review: consolidate gating test + add integration test
Collapse the gating shape checks into one parameterized
test_gating_variations, add an end-to-end per-element gate test, and a
slow integration test asserting golden logits on a tiny published
checkpoint (poolside/Laguna-tiny-per-element).
* small nits
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Co-authored-by: Vasqu <antonprogamer@gmail.com>
