MetaBackend

[BrunoLiegiBastonLiegi]

This PR implements a MetaBackend object that will call the different projects' (qibojit, qibolab, etc...) MetaBackend to load their backends. This will replace the current construct_backend function and will also give access to a list_available() function to list all the backends available in the system.

Closes #1096.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[BrunoLiegiBastonLiegi]

We have the same problem here as in #1239. Namely, since now in main qibo depends on qibotn/py3.12 and qibojit/py3.12, I cannot resolve the conflicts without breaking something. For qibojit there is a simple solution which is changing the qibo dependency to qibojit/main as the corresponding qibojit PR has already been merged, however for qibotn the py3.12 PR does not seem to be mergeable soon.

[alecandido]

True, this py3.12 upgrade is being a bit of a nightmare (I hope qibo-core will land soon...)

however for qibotn the py3.12 PR does not seem to be mergeable soon.

Here I disagree a bit, since qiboteam/qibotn#48 should be as good as main, and py3.12 compatible.
I just need the two approvals, thus I'm adding you as well as a reviewer :P

[alecandido]

@BrunoLiegiBastonLiegi the issue with Pylint 3.2 is not completely trivial, thus I opened #1328 for it.

At this point, it's not worth to stop this any longer. Feel free to merge whenever you wish :)

[BrunoLiegiBastonLiegi]

I was waiting for qibotn and qibo-cloud-backends to be merged before merging this one, just to avoid breaking the set_backend for those two.

[alecandido]

I was waiting for qibotn and qibo-cloud-backends to be merged before merging this one, just to avoid breaking the set_backend for those two.

If they are going to be merged this week, so much the better (we'll need one less upgrade for the pyproject.toml/poetry.lock).
However, it doesn't have to be broken for them, since it will keep working with a previous release (and, in practice, they are relying on Qibo internals, so fine to break in new versions - the MetaBackend is exactly for that, to depend less), and it will work again with the latest one as soon as the PRs will be merged.