[core] fix 4bit num_parameters (#26132)

* fix 4bit `num_parameters`

* stronger check

tests/quantization/bnb/test_4bit.py

@@ -118,6 +118,17 @@ class Bnb4BitTest(Base4bitTest):
         gc.collect()
         torch.cuda.empty_cache()
 
+    def test_quantization_num_parameters(self):
+        r"""
+        Test if the number of returned parameters is correct
+
+        See: https://github.com/huggingface/transformers/issues/25978
+        """
+        num_params_4bit = self.model_4bit.num_parameters()
+        num_params_fp16 = self.model_fp16.num_parameters()
+
+        self.assertEqual(num_params_4bit, num_params_fp16)
+
     def test_quantization_config_json_serialization(self):
         r"""
         A simple test to check if the quantization config is correctly serialized and deserialized