[ET-VK] Prevent hardswish decomposition by adding to ops_not_to_decompose

[SS-JIA]

Summary: Without this change, aten.hardswish.default decomposes into several
arithmetic ops (add, clamp, mul, div) which prevents the Vulkan conv2d
depthwise fusion pass from recognizing and fusing the activation. Adding
hardswish to ops_not_to_decompose keeps it as a single op so the conv2d
output tile shader can apply the clamp fusion.

Also adds the conv2d_dw_output_tile_3x3_b1x1_clamp shader variant needed
for the fused path.

Authored with Claude.

Test Plan: Exported MobileNetV3 for Vulkan and verified hardswish no longer decomposes.

Reviewers:

Subscribers:

Tasks:

Tags:

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Stack created with Sapling. Best reviewed with ReviewStack.

• [ET-VK] Only partition copy.default to Vulkan when it is a no-op #18576
• -> [ET-VK] Prevent hardswish decomposition by adding to ops_not_to_decompose #18575

[pytorch-bot]

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❌ 2 New Failures, 3 Unrelated Failures

As of commit 7adc046 with merge base fa81941 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner-mypy (gh)
  >>> Lint for backends/arm/test/models/stable_diffusion/stable_diffusion_module_test_configs.py:
• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t 64452c66198e5cdf107b5630f46736766e70495d38ff85884c6657b18e13c253 /exec failed with exit code 139

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

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

• pull / android / run-emulator (gh) (trunk failure)
  The process '/usr/local/lib/android/sdk/platform-tools/adb' failed with exit code 224
• 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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