Implementing Qibo Nexus backend

[jf-kong]

Summary

Initial implementation of nexus-backend, a standalone Qibo backend for compiling and executing circuits on Quantinuum hardware through qnexus API, modelled on the layout of qibojit and qibolab.

The Nexus client was previously developed inside qibo-cloud-backends (never released or merged into main). This PR carves the Nexus pieces out into their own home.

What's in this PR

• Source (src/nexus_backend/, 7 modules)
  • backend.py — NexusClientBackend (formerly nexus_client.py); name = "nexus-backend".
  • auth.py, config.py, errors.py, translation.py, results.py, helios.py — moved with sibling-import rewrites only.
  • __init__.py — exposes MetaBackend, NexusClientBackend, NexusBackendConfig, EstimateItem, ExecutionEstimate, run_compile_execute, PLATFORMS, __version__.
• Tests (tests/, 6 files, 50 tests, all green)
  • test_meta_backend.py covers the qibo.set_backend("nexus-backend", ...) discovery path.
  • test_backend_contract.py covers direct NexusClientBackend(...).execute_circuit/.execute_circuits/.estimate_circuit/.estimate_circuits against mocked qnexus.
  • test_translator.py, test_config.py, test_results.py, test_helios.py — unit tests for the translation, config, result-mapping, and Helios HUGR layers.
  • All tests fully mock qnexus, pytket, pytket.qasm, guppylang, guppylang.std.*, qibo.result via monkeypatch.setitem(sys.modules, …). No live Nexus calls; CI runs offline.
• Packaging (pyproject.toml)
  • Poetry-based, mirrors qibojit's shape.
  • Required deps: qibo ^0.2.16, qnexus >=0.39,<1, pytket >=2.8,<3, guppylang >=0.21,<1.
  • Optional groups: tests (pytest, pytest-cov, pytest-mock, pylint), docs (sphinx, furo, …).
• Docs (doc/source/)
  • conf.py
  • index.rst (install + quickstart) and api.rst (autoclass on NexusClientBackend and MetaBackend).
• CI (.github/workflows/)
  • unit_test.yml: Linux + macOS × Python 3.10/3.11/3.12, Poetry install, poe lint + poe test, Codecov upload on Linux 3.10.
  • deploy.yml: scaffolded but gated to workflow_dispatch only; PyPI publish is deferred.
• README.md: install (currently via pip install git+…), quickstart.

Additional note

qibo.set_backend("nexus-backend", platform=...) triggers qibo.backends.construct_backend(...), which substitutes - → _ in the backend string, calls importlib.import_module("nexus_backend"), and invokes module.MetaBackend.load(**kwargs). The contract this PR satisfies is therefore minimal: be importable as nexus_backend and expose a top-level MetaBackend.load returning a qibo.backends.abstract.Backend subclass. Same shape qibojit and qibolab use.

Verification done locally

• poetry install --with tests — resolves cleanly.
• poetry run pytest — 50 passed in 44s, fully offline, no Nexus credentials required.
• qibo.set_backend("nexus-backend", platform="hseries:H2-1LE") — returns a real NexusClientBackend (module=nexus_backend.backend, name=nexus-backend, config.platform=hseries:H2-1LE).
• from nexus_backend import NexusClientBackend; backend = NexusClientBackend(platform="...") — direct construction works as before; all execute_circuit / execute_circuits / estimate_circuit / estimate_circuits methods are reachable.
• grep -RIn "qibo_cloud_backends|nexus-client|qibo-cloud-backends" src tests doc README.md pyproject.toml — zero hits (no stale references).

Test plan for reviewers

• poetry install --with tests && poetry run pytest passes locally.
• CI matrix is green (Linux + macOS × Py 3.10/3.11/3.12).
• python -c "import qibo; qibo.set_backend('nexus-backend', platform='hseries:H2-1LE'); print(qibo.get_backend())" reports a NexusClientBackend with name == "nexus-backend".

[codecov]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️

[scarrazza]

Replace this file by using the workflow stored in https://github.com/qiboteam/workflows/blob/v2/.github/workflows/test.yml, see example from qibolab: https://github.com/qiboteam/qibolab/blob/main/.github/workflows/test.yml

[scarrazza]

Thanks @jf-kong, please find some comments here. Could you please check coverage? I think we can avoid the sphinx for now and just keep the docs in the README (so we can avoid the documentation workflow too). If we decide to upload to pypi then we can rethink about it.

[scarrazza]

Suggested change

	lint-warnings = "pylint --exit-zero src"
	lint-warnings = "pylint --exit-zero src"
	types = "true"