Add Python runtime end-to-end tests for XNNPACK on Linux

[Lidang-Jiang]

Summary

Add Python runtime end-to-end tests for the XNNPACK backend on Linux — the first batch for the testing gap described in #11225.

This update keeps the Linux-only coverage under runtime/test/test_runtime_xnnpack.py so the standard unittest jobs collect it cleanly. Each test exports a small model with XNNPACK delegation, saves it as .pte, loads it via executorch.runtime.Runtime, and compares the output against PyTorch eager mode.

Models tested:

• test_add — element-wise addition
• test_linear — single Linear layer (fp32)
• test_conv2d_relu — Conv2d + ReLU
• test_mlp — multi-layer perceptron (Linear -> ReLU -> Linear)
• test_depthwise_separable_conv — depthwise + pointwise conv (MobileNet building block)
• test_classifier_head — AdaptiveAvgPool2d + Flatten + Linear
• test_conv_bn — Conv2d + BatchNorm2d + ReLU (skipped: FuseBatchNormPass crashes; TODO to re-enable)

Part of #11225 (Linux XNNPACK)

Before

$ pull / unittest-editable / linux / linux-job

==================================== ERRORS ====================================
_________ ERROR collecting test/end2end/test_python_runtime_xnnpack.py _________
ImportError while importing test module '/pytorch/executorch/test/end2end/test_python_runtime_xnnpack.py'.
Traceback:
E   ModuleNotFoundError: No module named 'test.end2end'

$ lintrunner --skip MYPY test/end2end/test_python_runtime_xnnpack.py

Warning (UFMT) format
- from executorch.backends.xnnpack.partition.xnnpack_partitioner import (
-     XnnpackPartitioner,
- )
+ from executorch.backends.xnnpack.partition.xnnpack_partitioner import XnnpackPartitioner

After

$ lintrunner --skip MYPY runtime/test/test_runtime_xnnpack.py
ok No lint issues.

$ python -m py_compile runtime/test/test_runtime_xnnpack.py

Test plan

• Moved the Linux-only XNNPACK runtime tests to runtime/test/test_runtime_xnnpack.py
• Added runtime/test/TARGETS coverage for the new Python runtime test
• lintrunner --skip MYPY runtime/test/test_runtime_xnnpack.py
• python -m py_compile runtime/test/test_runtime_xnnpack.py
• test_conv_bn remains skipped with TODO — FuseBatchNormPass still has a known bug on Sequential(Conv2d, BN) export
• Full python -m pytest runtime/test/test_runtime_xnnpack.py -v
  Local blocker: this workstation does not currently have a fully provisioned ExecuTorch runtime test environment; upstream CI remains the source of truth for this suite.

cc @GregoryComer @digantdesai @cbilgin @jathu @larryliu0820

This PR was authored with the help of Claude.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18703

• 📄 Preview Python docs built from this PR

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✅ You can merge normally! (2 Unrelated Failures)

As of commit 7e51933 with merge base ac68932 ():

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.

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[Lidang-Jiang]

@pytorchbot label "release notes: xnnpack"

[GregoryComer]

@Lidang-Jiang Can you resolve the lint - https://github.com/pytorch/executorch/actions/runs/24114688479/job/71164744422?pr=18703? I can go ahead and merge once CI is green. I ended up closing the original issue, as it's old, but adding more test coverage here doesn't hurt. Thanks!

[Lidang-Jiang]

@GregoryComer Fixed in e1b05d5. I moved the Linux-only XNNPACK Python runtime tests into runtime/test so they collect cleanly in the standard unittest jobs, and I fixed the UFMT lint issue.