[ET-VK][conv2d_dw] Extract depthwise dispatch into Conv2dDW.cpp with device-based tile selection

[SS-JIA]

Stack from ghstack (oldest at bottom):

• -> [ET-VK][conv2d_dw] Extract depthwise dispatch into Conv2dDW.cpp with device-based tile selection #18293
• [ET-VK][conv2d] Re-implement pointwise conv2d with tiled compute and blocked weight packing #18292
• [ET-VK] Fix staging buffer allocation to check all memory types for HOST_CACHED #18291

Profiling showed depthwise conv2d is 5-15x slower on Mali GPUs vs Adreno due to
register pressure from the 4x2 output tile (17 vec4 registers per thread).
Benchmarking confirmed that reducing the tile to 1x1 (7 vec4 registers) gives
4-15x speedup on Mali with no regression on Adreno.

This change extracts depthwise conv2d dispatch logic from Convolution.cpp into a
new Conv2dDW.cpp (following the Conv2dPW.cpp pattern), and adds device-based
tile size selection: b1x1 on Mali, b4x2 (current default) on Adreno.

Differential Revision: D97058158

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18293

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 1 New Failure, 2 Unrelated Failures

As of commit f5c972c with merge base ed57040 ():

NEW FAILURE - The following job has failed:

• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t 08adc0e3411993b7d6ad86e683e715f37e51c40dbdd76be5ac9e98a4b0669ecb /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 / 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.

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[github-actions]

This PR needs a release notes: label

If your change should be included in the release notes (i.e. would users of this library care about this change?), please use a label starting with release notes:. This helps us keep track and include your important work in the next release notes.

To add a label, you can comment to pytorchbot, for example
@pytorchbot label "release notes: none"

For more information, see
https://github.com/pytorch/pytorch/wiki/PyTorch-AutoLabel-Bot#why-categorize-for-release-notes-and-how-does-it-work.