Fix FlaxPretTrainedModel pt weights check (huggingface#19133)

* Fix FlaxPretTrainedModel pt weights check

* Update src/transformers/modeling_flax_utils.py

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* fix raise comment

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

src/transformers/modeling_flax_utils.py

@@ -665,7 +665,7 @@ def from_pretrained(
                     archive_file = os.path.join(pretrained_model_name_or_path, FLAX_WEIGHTS_INDEX_NAME)
                     is_sharded = True
                 # At this stage we don't have a weight file so we will raise an error.
-                elif os.path.join(pretrained_model_name_or_path, WEIGHTS_NAME):
+                elif os.path.isfile(os.path.join(pretrained_model_name_or_path, WEIGHTS_NAME)):
                     raise EnvironmentError(
                         f"Error no file named {FLAX_WEIGHTS_NAME} found in directory {pretrained_model_name_or_path} "
                         "but there is a file for PyTorch weights. Use `from_pt=True` to load this model from those "