Arm backend: Use bucket approach in fuse_equal_placeholder_pass

[Erik-Lundell]

Verified to catch same dupes as before in lstm and mv2.

Instead of comparing all placeholders to each other, compute a hash use as key in dictionary. Equal placeholder -> equal key. If an entry in the dictionary has multiple values, we have duplicates.

This is a ~O(N) algorithm compared to earlier O(N^2). This can be seen by measuring the speedup for lstm vs. mv2

lstm: 120 placeholders (116 dupes) 0.4s -> 0.3s

mv2: 318 placeholders (98 dupes) ~15s -> 0.5s

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13271

• 📄 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 Cancelled Jobs, 1 Unrelated Failure

As of commit cdadc5e with merge base 72580d2 ():

NEW FAILURE - The following job has failed:

• trunk / test-models-macos-cpu (w2l, xnnpack-quantization-delegation) / macos-job (gh)
  The process '/usr/bin/git' failed with exit code 128

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• trunk / test-models-macos-coreml (dl3) / macos-job (gh)
  ##[error]The operation was canceled.
• trunk / test-models-macos-cpu (efficient_sam, portable) / macos-job (gh)
  ##[error]The operation was canceled.

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

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

• Apple / build-demo-ios / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 65

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

[Erik-Lundell]

Test failures unrelated.