Minor speedup for model lowering: Skip redundant run_decompositions when no ops match decomp table (#18496)

[apullin]

Summary:

Adds an early-exit check to _gen_edge_manager_for_partitioners: before
calling program.run_decompositions(table), scan the graph for ops that
appear in the decomposition table. If none are found, skip the call
entirely.

Each run_decompositions call performs a full re-export of the program
via make_fx(), re-tracing every node through FakeTensor dispatch.
On the EDGE_DO_NOT_DECOMP path this function is called up to 3 times;
the early-exit eliminates at least one redundant call where the previous
pass already decomposed all matching ops.

The check recursively walks control flow submodules (cond/map/scan) to
avoid incorrectly skipping when decomposable ops are nested.

Benchmark

Model: small CNN feature extractor (~50K params, 9 conv layers with
LayerNorm, targeting Ethos-U55 via the ARM/TOSA lowering pipeline).
Graph: ~1200 nodes.

lower() before: 82 s
lower() after: 71 s
Delta: -11 s (-13 %)

Differential Revision: D96489903

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18496

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❌ 5 New Failures, 6 Unrelated Failures, 5 Unclassified Failures

As of commit 33061a2 with merge base 55c54c7 ():

NEW FAILURES - The following jobs have failed:

• pull / test-arm-backend-no-driver (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t ce431c068c650d8b8021175e04839c79617b1513b6450e41420a5632c562ca74 /exec failed with exit code 1
• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t b542fb8183da035957495463ed19691f4908e382a4447f0f571e94a34320b091 /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 48a71f7bbe51e9401e9db0d9995581c4148107106b2cde160932a966c42a9ec6 /exec failed with exit code 1
• trunk / test-arm-backend-ethos-u (test_pytest_ops_ethos_u85) / linux-job (gh)
  RuntimeError: Command docker exec -t 7f9f2f78755dc4364c7c9b58f97030c7fc089d26607599cef667aa6e5908291a /exec failed with exit code 1
• trunk / unittest-release / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 789c7ee217f9a11c86c77a0af12114058411b5aa1b044736e40cc14369d9fcba /exec failed with exit code 1

UNCLASSIFIED FAILURES - DrCI could not classify the following jobs because the workflow did not run on the merge base. The failures may be pre-existing on trunk or introduced by this PR:

• trunk / test-models-macos-cpu (ic4, xnnpack-quantization-delegation) / macos-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: operator torchvision::nms does not exist
• trunk / test-models-macos-cpu (llama3_2_vision_encoder, portable) / macos-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos-cpu (mobilebert, xnnpack-quantization-delegation) / macos-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos-cpu (mv3, portable) / macos-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos-cpu (vit, xnnpack-quantization-delegation) / macos-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: operator torchvision::nms does not exist

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

• trunk / test-models-macos-mps / macos-job (gh) (similar failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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

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

• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)
• pull / unittest / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)
• trunk / unittest-release / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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

[meta-codesync]

@apullin has exported this pull request. If you are a Meta employee, you can view the originating Diff in D96489903.

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