[core / Quantization] Fix for 8bit serialization tests (#27234)

* fix for 8bit serialization

* added regression tests.

* fixup

tests/quantization/bnb/test_mixed_int8.py

@@ -369,6 +369,33 @@ class MixedInt8Test(BaseMixedInt8Test):
                 self.tokenizer.decode(output_sequences[0], skip_special_tokens=True), self.EXPECTED_OUTPUT
             )
 
+    def test_int8_serialization_regression(self):
+        r"""
+        Test whether it is possible to serialize a model in 8-bit - using not safetensors
+        """
+        from bitsandbytes.nn import Int8Params
+
+        with tempfile.TemporaryDirectory() as tmpdirname:
+            self.model_8bit.save_pretrained(tmpdirname, safe_serialization=False)
+
+            # check that the file `quantization_config` is present
+            config = AutoConfig.from_pretrained(tmpdirname)
+            self.assertTrue(hasattr(config, "quantization_config"))
+
+            model_from_saved = AutoModelForCausalLM.from_pretrained(tmpdirname, load_in_8bit=True, device_map="auto")
+
+            linear = get_some_linear_layer(model_from_saved)
+            self.assertTrue(linear.weight.__class__ == Int8Params)
+            self.assertTrue(hasattr(linear.weight, "SCB"))
+
+            # generate
+            encoded_input = self.tokenizer(self.input_text, return_tensors="pt")
+            output_sequences = model_from_saved.generate(input_ids=encoded_input["input_ids"].to(0), max_new_tokens=10)
+
+            self.assertEqual(
+                self.tokenizer.decode(output_sequences[0], skip_special_tokens=True), self.EXPECTED_OUTPUT
+            )
+
     def test_int8_serialization_sharded(self):
         r"""
         Test whether it is possible to serialize a model in 8-bit - sharded version.