Fix flaky ONNX tests (#6531)

tests/test_onnx.py

 import unittest
-from os.path import dirname, exists
 from pathlib import Path
-from shutil import rmtree
 from tempfile import NamedTemporaryFile, TemporaryDirectory
 
 from transformers import BertConfig, BertTokenizerFast, FeatureExtractionPipeline
@@ -72,7 +70,7 @@ class OnnxExportTestCase(unittest.TestCase):
     def test_quantize_pytorch(self):
         for model in OnnxExportTestCase.MODEL_TO_TEST:
             path = self._test_export(model, "pt", 12)
-            quantized_path = quantize(Path(path))
+            quantized_path = quantize(path)
 
             # Ensure the actual quantized model is not bigger than the original one
             if quantized_path.stat().st_size >= Path(path).stat().st_size:
@@ -82,11 +80,11 @@ class OnnxExportTestCase(unittest.TestCase):
         try:
             # Compute path
             with TemporaryDirectory() as tempdir:
-                path = tempdir + "/model.onnx"
+                path = Path(tempdir).joinpath("model.onnx")
 
             # Remove folder if exists
-            if exists(dirname(path)):
-                rmtree(dirname(path))
+            if path.parent.exists():
+                path.parent.rmdir()
 
             # Export
             convert(framework, model, path, opset, tokenizer)