closing Local backend - bug

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.

Start by locating art.backend.Backend.close() and art.local.backend.LocalBackend.close(), then inspect how _wandb_runs and _weave_clients are managed. Confirm the close path that fails when awaited, and verify that completion of a LocalBackend also finishes tracked runs and clients so their results are flushed.