Add YOCO KV sharing support to StaticAttention (#18517)

[billmguo]

Summary:

Add YOCO (You Only Cache Once) support to StaticAttention so models with
num_kv_shared_layers > 0 can be transformed via
transform_attention_mha_to_static_attention and run correctly through the
static attention export pipeline.

Changes:

• StaticAttention.init: skip wks/wvs/caches for shared layers
• from_attention_mha: detect shared layers, preserve LoRA on wq/wo
• forward/_forward_mha/_forward_sha: accept shared_kv, skip K/V projection
• StaticAttentionIOManager: skip cache allocation for shared layers
• Tests: 6 new test methods covering shared/donor layers, LoRA, numerics

When num_kv_shared_layers=0 (default), behavior is completely unchanged.
No C++ runtime changes needed.

Reviewed By: limintang, YIWENX14

Differential Revision: D97556018

[pytorch-bot]

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❌ 1 New Failure, 2 Unrelated Failures

As of commit e08f97d with merge base b6824d1 ():

NEW FAILURE - The following job has failed:

• pull / unittest-arm-backend-with-no-deps (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 249469fbc7e9e902a5745e71d75e40f3e63eafb54edee7acdbf11f068d79a5f1 /exec failed with exit code 1

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[meta-codesync]

@billmguo has exported this pull request. If you are a Meta employee, you can view the originating Diff in D97556018.

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