fix critic save error (#199)

This PR is similar to PR #174 but
fix the critic save error

I move the old PR to this one due to some redundant commit

verl/workers/fsdp_workers.py

@@ -816,7 +816,7 @@ def save_checkpoint(self, local_path, hdfs_path=None):
             if hdfs_path is not None:
                 print(f'Uploading critic checkpoint to {hdfs_path}')
                 hdfs_io.makedirs(hdfs_path, exist_ok=True)
-                hdfs_io.copy(src=local_path, dst=hdfs_path)
+                hdfs_io.copy(src=local_path, dst=hdfs_path, dirs_exist_ok=True)
 
         torch.distributed.barrier()
         if self._is_offload_param: