FIX [Trainer / tags]: Fix trainer + tags when users do not pass "tags" to trainer.push_to_hub() (#29009)

* fix trainer tags

* add test

src/transformers/trainer.py

@@ -3842,7 +3842,10 @@ class Trainer:
         # Add additional tags in the case the model has already some tags and users pass
         # "tags" argument to `push_to_hub` so that trainer automatically handles internal tags
         # from all models since Trainer does not call `model.push_to_hub`.
-        if "tags" in kwargs and getattr(self.model, "model_tags", None) is not None:
+        if getattr(self.model, "model_tags", None) is not None:
+            if "tags" not in kwargs:
+                kwargs["tags"] = []
+
             # If it is a string, convert it to a list
             if isinstance(kwargs["tags"], str):
                 kwargs["tags"] = [kwargs["tags"]]

tests/trainer/test_trainer.py

@@ -30,7 +30,7 @@ from typing import Dict, List
 from unittest.mock import Mock, patch
 
 import numpy as np
-from huggingface_hub import HfFolder, delete_repo, list_repo_commits, list_repo_files
+from huggingface_hub import HfFolder, ModelCard, delete_repo, list_repo_commits, list_repo_files
 from parameterized import parameterized
 from requests.exceptions import HTTPError
 
@@ -2564,7 +2564,13 @@ class TrainerIntegrationWithHubTester(unittest.TestCase):
 
     @classmethod
     def tearDownClass(cls):
-        for model in ["test-trainer", "test-trainer-epoch", "test-trainer-step", "test-trainer-tensorboard"]:
+        for model in [
+            "test-trainer",
+            "test-trainer-epoch",
+            "test-trainer-step",
+            "test-trainer-tensorboard",
+            "test-trainer-tags",
+        ]:
             try:
                 delete_repo(token=cls._token, repo_id=model)
             except HTTPError:
@@ -2695,6 +2701,31 @@ class TrainerIntegrationWithHubTester(unittest.TestCase):
 
         assert found_log is True, "No tensorboard log found in repo"
 
+    def test_push_to_hub_tags(self):
+        # Checks if `trainer.push_to_hub()` works correctly by adding the desired
+        # tag without having to pass `tags` in `push_to_hub`
+        # see:
+        with tempfile.TemporaryDirectory() as tmp_dir:
+            trainer = get_regression_trainer(
+                output_dir=os.path.join(tmp_dir, "test-trainer-tags"),
+                push_to_hub=True,
+                hub_token=self._token,
+            )
+
+            trainer.model.add_model_tags(["test-trainer-tags"])
+
+            url = trainer.push_to_hub()
+
+            # Extract repo_name from the url
+            re_search = re.search(ENDPOINT_STAGING + r"/([^/]+/[^/]+)/", url)
+            self.assertTrue(re_search is not None)
+            repo_name = re_search.groups()[0]
+
+            self.assertEqual(repo_name, f"{USER}/test-trainer-tags")
+
+            model_card = ModelCard.load(repo_name)
+            self.assertTrue("test-trainer-tags" in model_card.data.tags)
+
 
 @require_torch
 @require_optuna