Fix flaky `test_for_warning_if_padding_and_no_attention_mask` (#24706)

fix
Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>

tests/models/bert/test_modeling_bert.py

@@ -584,6 +584,9 @@ class BertModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineTesterMixin
 
         # Check for warnings if the attention_mask is missing.
         logger = logging.get_logger("transformers.modeling_utils")
+        # clear cache so we can test the warning is emitted (from `warning_once`).
+        logger.warning_once.cache_clear()
+
         with CaptureLogger(logger) as cl:
             model = BertModel(config=config)
             model.to(torch_device)