[ET-VK][runtime][2/3] Add typed dispatch size abstractions

[SS-JIA]

Stack from ghstack (oldest at bottom):

• [ET-VK][ops] Add batch support to q8ta convolutions #22254
• [ET-VK][runtime][3/3] Enforce device-safe dispatch geometry #22225
• -> [ET-VK][runtime][2/3] Add typed dispatch size abstractions #22224
• [ET-VK][runtime][1/3] Prepare linear shaders for 2D dispatch #22223
• [ET-VK][ops] Move group norm reduction lanes to Z #22222

Introduce LocalWorkGroup, LwgShape, GlobalWorkGrid, and
DispatchGridIntent in runtime/vk_api/DispatchGrid. Remove legacy
WorkgroupSize.

Model local shapes as power-of-two exponents, support fitting against global
extents, and validate local dimensions against device limits. Track dispatch
intent independently from optional required local workgroup geometry.

Authored with Codex.

Differential Revision: D117706351

[pytorch-bot]

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❌ 1 New Failure, 1 Unrelated Failure

As of commit 43da1f8 with merge base a5f15b5 ():

NEW FAILURE - The following job has failed:

• pull / test-moshi-linux / linux-job (gh)
  RuntimeError: Command docker exec -t fdfb0f7aa1d81d73961475a351c4e17639b75921172b807419f15f1e112e5e76 /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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