add pathlib (#4395)

nni/compression/pytorch/speedup/compressor.py

 # Copyright (c) Microsoft Corporation.
 # Licensed under the MIT license.
-import os
-import queue
-import logging
+
 import copy
+import logging
+from pathlib import Path
+import queue
+
 import torch
 import torch.nn as nn
 
@@ -63,7 +65,7 @@ class ModelSpeedup:
         # load the mask tensor to the same device with the dummy_input
         # self.masks save the mask tensors pruned by the user and the infered
         # masks of the others modules
-        if isinstance(masks_file, str) and os.path.exists(masks_file):
+        if isinstance(masks_file, (str, Path)) and Path(masks_file).exists():
             self.masks = torch.load(
                 masks_file, map_location if map_location is not None else str(self.device))
         elif isinstance(masks_file, dict):