[bnb] Let's warn users when saving 8-bit models (#20282)

* add warning on 8-bit models

- added tests
- added wrapper

* move to a private attribute

- remove wrapper
- changed `save_pretrained` method

* Apply suggestions from code review

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

* fix suggestions

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

tests/mixed_int8/test_mixed_int8.py

@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import gc
+import tempfile
 import unittest
 
 from transformers import (
@@ -107,6 +108,13 @@ class MixedInt8Test(BaseMixedInt8Test):
 
         self.assertEqual(self.tokenizer.decode(output_sequences[0], skip_special_tokens=True), self.EXPECTED_OUTPUT)
 
+    def test_warns_save_pretrained(self):
+        r"""
+        Test whether trying to save a model after converting it in 8-bit will throw a warning.
+        """
+        with self.assertWarns(UserWarning), tempfile.TemporaryDirectory() as tmpdirname:
+            self.model_8bit.save_pretrained(tmpdirname)
+
 
 class MixedInt8ModelClassesTest(BaseMixedInt8Test):
     def setUp(self):