add destination path to client artifacts download (mlflow#1504)

mlflow/tracking/client.py

@@ -234,19 +234,24 @@ def list_artifacts(self, run_id, path=None):
         artifact_repo = get_artifact_repository(artifact_root)
         return artifact_repo.list_artifacts(path)
 
-    def download_artifacts(self, run_id, path):
+    def download_artifacts(self, run_id, path, dst_path=None):
         """
         Download an artifact file or directory from a run to a local directory if applicable,
         and return a local path for it.
 
         :param run_id: The run to download artifacts from.
         :param path: Relative source path to the desired artifact.
+        :param dst_path: Absolute path of the local filesystem destination directory to which to
+                         download the specified artifacts. This directory must already exist.
+                         If unspecified, the artifacts will either be downloaded to a new
+                         uniquely-named directory on the local filesystem or will be returned
+                         directly in the case of the LocalArtifactRepository.
         :return: Local path of desired artifact.
         """
         run = self.get_run(run_id)
         artifact_root = run.info.artifact_uri
         artifact_repo = get_artifact_repository(artifact_root)
-        return artifact_repo.download_artifacts(path)
+        return artifact_repo.download_artifacts(path, dst_path)
 
     def set_terminated(self, run_id, status=None, end_time=None):
         """Set a run's status to terminated.