fix: MLFlowLogger artifact_path (#20669)

* fix: MLFlowLogger artifact_path

* fix: use `as_posix()`

Author: yxtay

Diff for: src/lightning/pytorch/loggers/mlflow.py

@@ -363,7 +363,7 @@ def _scan_and_log_checkpoints(self, checkpoint_callback: ModelCheckpoint) -> Non
             aliases = ["latest", "best"] if p == checkpoint_callback.best_model_path else ["latest"]
 
             # Artifact path on mlflow
-            artifact_path = Path(self._checkpoint_path_prefix) / Path(p).stem
+            artifact_path = Path(self._checkpoint_path_prefix, Path(p).stem).as_posix()
 
             # Log the checkpoint
             self.experiment.log_artifact(self._run_id, p, artifact_path)