Explicitly convert artifact path to posix_path (ultralytics#3067)

* Explicitly convert artifact path to posix_path

* Remove redudant str() casting

(cherry picked from commit d2a1728)

utils/wandb_logging/wandb_utils.py

@@ -159,7 +159,8 @@ def setup_training(self, opt, data_dict):
 
     def download_dataset_artifact(self, path, alias):
         if isinstance(path, str) and path.startswith(WANDB_ARTIFACT_PREFIX):
-            dataset_artifact = wandb.use_artifact(remove_prefix(path, WANDB_ARTIFACT_PREFIX) + ":" + alias)
+            artifact_path = Path(remove_prefix(path, WANDB_ARTIFACT_PREFIX) + ":" + alias)
+            dataset_artifact = wandb.use_artifact(artifact_path.as_posix())
             assert dataset_artifact is not None, "'Error: W&B dataset artifact doesn\'t exist'"
             datadir = dataset_artifact.download()
             return datadir, dataset_artifact