build: record physical plugin split experiment

[voltjia]

Summary

Record a physical split experiment using an external plugin root outside the main repository.

This PR is stacked on #642 (build/manifest-cmake-entry).

Changes

• Add docs/plugin_physical_split_experiment.md with the exact external plugin layout, configure/build commands, generated registry evidence, and findings.
• Demonstrate a split external-cpu-add plugin under /tmp/infini-ops-physical-split/plugins that contributes an external ops/add/add.h implementation to one libinfiniops.so build.
• Capture the current limitation that generated sources include absolute external paths, which works for local builds but should be hardened before long-term physical split support.

Validation

• .venv/bin/python -m pytest -s -q tests/test_plugin_registry.py tests/test_plugin_test_matrix.py tests/test_generate_wrappers_plugins.py
• .venv/bin/ruff check scripts/infini_ops_plugin_registry.py scripts/infini_ops_plugin_test_matrix.py scripts/generate_wrappers.py tests/test_plugin_registry.py tests/test_plugin_test_matrix.py tests/test_generate_wrappers_plugins.py
• .venv/bin/python -m py_compile scripts/infini_ops_plugin_registry.py scripts/infini_ops_plugin_test_matrix.py scripts/generate_wrappers.py
• git diff --check
• Physical split external plugin configure/codegen/build smoke

Full-platform validation

Full stack validation was run on the top branch build/external-plugin-source-paths at pre-style-rebase commit 907eff70; see #643 validation comment: #643 (comment). The later rebase only updates PR metadata and diagnostic wording for CONTRIBUTING.md compliance.

• NVIDIA: build passed; pytest failed with 1 failed, 3687 passed, 4404 skipped due to CUDA OOM in tests/test_torch_ops.py::test_op[..., svd] on the PyTorch reference path.
• MetaX: 3183 passed, 3400 skipped.
• Iluvatar: 2689 passed, 3894 skipped.
• Moore: passed after preloading the container OpenMP runtime, 2949 passed, 3643 skipped.
• Cambricon: 1781 passed, 4694 skipped.
• Ascend: pytest passed with 3359 passed, 3233 skipped, but the container exited with code 137; treat as not fully green by the quality gate.

[voltjia]

Full-Platform Validation for build/external-plugin-source-paths

Validated commit 907eff70 from PR #643 across the requested platform set. Source was synced to remote scratch directories and each platform ran a fresh wheel build plus full pytest -v with JUnit output in the platform CI container. PyTorch backend was enabled by the platform build auto-detection, and tests/test_torch_ops.py was included in collection/execution on all platforms.

Platform	Build	pytest result	Time	Notes
NVIDIA	Passed	Failed: 1 failed, 3687 passed, 4404 skipped	build 992s, test log summary 25s	Failure is tests/test_torch_ops.py::test_op[..., svd]; log shows CUDA OOM during the PyTorch reference path.
MetaX	Passed	Passed: 3183 passed, 3400 skipped	build 1377s, test 87s	Full pytest completed with exit code 0.
Iluvatar	Passed	Passed: 2689 passed, 3894 skipped	build 704s, test 40s	First attempt hit multi-backend auto-detect (WITH_NVIDIA + WITH_ILUVATAR); rerun used explicit WITH_ILUVATAR=ON and passed.
Moore	Passed	Passed: 2949 passed, 3643 skipped	build 2070s, test 45s	First attempt hit missing OpenMP runtime at import (__kmpc_for_static_fini); rerun preloaded the MUSA OpenMP runtime and passed.
Cambricon	Passed	Passed: 1781 passed, 4694 skipped	build 2209s, test 33s	Full pytest completed with exit code 0.
Ascend	Passed	Pytest passed: 3359 passed, 3233 skipped; container exit abnormal	build 1053s, test 61s	Reproduced twice: pytest completed and wrote JUnit, but Docker returned exit code 137 after the successful pytest summary, including a rerun without Docker memory limit. Not marked fully green because the process exit code is not 0.

Notes:

• No exact accelerator IDs are included here.
• NVIDIA and Ascend are findings under the quality gate: NVIDIA has a real pytest failure; Ascend has a successful pytest summary but non-zero container exit code.
• The plugin stack itself built on all platforms that reached pytest; no manifest/plugin-registry/codegen-specific failure was observed in these logs.