[ExecuTorch][Vulkan] Support scoped Gemma symbolic partitioning

[JCNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] Add Gemma 4 MTP and speculative-decode source-closure tests #21678
• [ExecuTorch][WebGPU] Add Gemma 4 speculative decode runtime #21677
• [ExecuTorch][WebGPU] Add Gemma 4 MTP export path #21676
• [ExecuTorch][WebGPU] Add Gemma 4 MTP operator and route support #21675
• [ExecuTorch][WebGPU] Add plain Gemma 4 source-closure tests #21674
• [ExecuTorch][WebGPU] Add Gemma 4 plain export and artifact contract #21673
• [ExecuTorch][WebGPU] Add Gemma 4 plain runtime and guarded routes #21672
• -> [ExecuTorch][Vulkan] Support scoped Gemma symbolic partitioning #21671
• [ExecuTorch][WebGPU] Add shared model runtime prerequisites #21670

Enable Gemma symbolic and fused export contracts without broadening global Vulkan support. Gemma export needs guard-only symbolic expressions, instance-scoped operator features, Hugging Face RoPE/SDPA rewrites, and safe reuse of already-mutated 8da4w parameters. These changes stay in the Vulkan serialization/export layer used by WebGPU and do not globally register Gemma-only operators.

Key changes:

• VulkanSupportedOperators accepts caller-owned extra_op_features and recognizes only symbolic chains that terminate exclusively in guard sinks.
• Vulkan pattern passes preserve Gemma single-HF-RoPE, SDPA, RMSNorm, select_as_symint, and shared 8da4w parameter contracts.
• Partitioner tests distinguish guard-only chains from live tensor consumers and verify instance-scoped features do not mutate the global registry.

Ordinary Vulkan partitioners retain their existing registry and compatibility behavior.

Co-authored-with: Claude Code.

Differential Revision: D115234083

[pytorch-bot]

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✅ You can merge normally! (2 Unrelated Failures)

As of commit 3efa072 with merge base ceca90f ():

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

• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, models) / linux-job (gh) (similar failure)
  /pytorch/executorch/backends/webgpu/runtime/ops/to_copy/ToCopy.cpp:207:6: error: redefinition of 'add_bool_to_float_op'

BROKEN TRUNK - The following job failed but was present on the merge base:

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

• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, operators) / linux-job (gh) (trunk failure)
  /pytorch/executorch/backends/webgpu/runtime/ops/to_copy/ToCopy.cpp:207:6: error: redefinition of 'add_bool_to_float_op'

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