Don't allow passing load_in_8bit and load_in_4bit at the same time (#28266)

* Update quantization_config.py

* Style

* Protect from setting directly

* add tests

* Update tests/quantization/bnb/test_4bit.py

Co-authored-by: Younes Belkada <49240599+younesbelkada@users.noreply.github.com>

---------

Co-authored-by: Younes Belkada <49240599+younesbelkada@users.noreply.github.com>

tests/quantization/bnb/test_4bit.py

@@ -648,3 +648,18 @@ class GPTSerializationTest(BaseSerializationTest):
     """
 
     model_name = "gpt2-xl"
+
+
+@require_bitsandbytes
+@require_accelerate
+@require_torch_gpu
+@slow
+class Bnb4BitTestBasicConfigTest(unittest.TestCase):
+    def test_load_in_4_and_8_bit_fails(self):
+        with self.assertRaisesRegex(ValueError, "load_in_4bit and load_in_8bit are both True"):
+            AutoModelForCausalLM.from_pretrained("facebook/opt-125m", load_in_4bit=True, load_in_8bit=True)
+
+    def test_set_load_in_8_bit(self):
+        quantization_config = BitsAndBytesConfig(load_in_4bit=True)
+        with self.assertRaisesRegex(ValueError, "load_in_4bit and load_in_8bit are both True"):
+            quantization_config.load_in_8bit = True