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>

src/transformers/utils/quantization_config.py

@@ -212,8 +212,12 @@ class BitsAndBytesConfig(QuantizationConfigMixin):
         **kwargs,
     ):
         self.quant_method = QuantizationMethod.BITS_AND_BYTES
-        self.load_in_8bit = load_in_8bit
-        self.load_in_4bit = load_in_4bit
+
+        if load_in_4bit and load_in_8bit:
+            raise ValueError("load_in_4bit and load_in_8bit are both True, but only one can be used at the same time")
+
+        self._load_in_8bit = load_in_8bit
+        self._load_in_4bit = load_in_4bit
         self.llm_int8_threshold = llm_int8_threshold
         self.llm_int8_skip_modules = llm_int8_skip_modules
         self.llm_int8_enable_fp32_cpu_offload = llm_int8_enable_fp32_cpu_offload
@@ -232,6 +236,26 @@ class BitsAndBytesConfig(QuantizationConfigMixin):
 
         self.post_init()
 
+    @property
+    def load_in_4bit(self):
+        return self._load_in_4bit
+
+    @load_in_4bit.setter
+    def load_in_4bit(self, value: bool):
+        if self.load_in_8bit and value:
+            raise ValueError("load_in_4bit and load_in_8bit are both True, but only one can be used at the same time")
+        self._load_in_4bit = value
+
+    @property
+    def load_in_8bit(self):
+        return self._load_in_8bit
+
+    @load_in_8bit.setter
+    def load_in_8bit(self, value: bool):
+        if self.load_in_4bit and value:
+            raise ValueError("load_in_4bit and load_in_8bit are both True, but only one can be used at the same time")
+        self._load_in_8bit = value
+
     def post_init(self):
         r"""
         Safety checker that arguments are correct - also replaces some NoneType arguments with their default values.

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