[plugin] torch ddp plugin supports sharded model checkpoint (#3775)

* [plugin] torch ddp plugin add save sharded model

* [test] fix torch ddp ckpt io test

* [test] fix torch ddp ckpt io test

* [test] fix low level zero plugin test

* [test] fix low level zero plugin test

* [test] add debug info

* [test] add debug info

* [test] add debug info

* [test] add debug info

* [test] add debug info

* [test] fix low level zero plugin test

* [test] fix low level zero plugin test

* [test] remove debug info

colossalai/checkpoint_io/checkpoint_io_base.py

@@ -1,7 +1,6 @@
 from abc import ABC, abstractmethod
 from pathlib import Path
-from typing import Union
-from typing import Optional
+from typing import Optional, Union
 
 import torch
 import torch.nn as nn
@@ -84,9 +83,8 @@ class CheckpointIO(ABC):
         # containing no distributed tensors, dtensor -> full tensor conversion
         # should be done offline via our CLI
         # the existence of index file means it is a sharded checkpoint
-        ckpt_path = Path(checkpoint)
         index_file_exists, index_file_path = has_index_file(checkpoint)
-        
+
         # return the origin model instead of the unwrapped model
         origin_model = model