build: support external plugin roots

[voltjia]

Summary

Allow builds and helper scripts to consume additional plugin roots, preparing for plugins hosted outside the main repository.

This PR is stacked on #639 (ci/plugin-test-matrix).

Changes

• Add INFINI_OPS_PLUGIN_ROOTS as comma- or semicolon-separated extra plugin roots.
• Keep INFINI_OPS_PLUGIN_ROOT as the built-in plugin root for compatibility.
• Update CMake plugin lookup to search multiple roots and reject duplicate plugin entries.
• Update Python registry loading and matrix logic to accept multiple roots.
• Document external plugin root selection in docs/plugin_contract.md.

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
• External plugin root CMake configure/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.