closing Local backend - bug

[giladfrid009]

version: openpipe-art 0.4.4

Notice that in the base class art.backend.Backend the functinon close() is async.
On the other hand, for the art.local.backend.LocalBackend backend, the function close() is not async.

As result, when executing for example await model.backend().close() an exception is getting thrown as we're awaiting a non-awaitable object.

---

Also related to closing a backend - it might be a good idea that upon closing a LocalBackend, all the runs in LocalBackend._wandb_runs and LocalBackend._weave_clients will be finished (and therefore their results are flushed), by calling:

for run in self._wandb_runs:
    run.finish()

and the same for the weave clients.

[arcticfly]

@giladfrid009 good catches! If you have time to make a draft PR of the changes you've proposed, we'd love the help! Otherwise someone from our team can get to it soon :)