FIX Prevent __getstate__ from mutating Params4bit

As discussed internally, use state = self.__dict__.copy(), which is also
what the Python docs recommend.

bitsandbytes/nn/modules.py

@@ -236,7 +236,7 @@ class Params4bit(torch.nn.Parameter):
         return self
 
     def __getstate__(self):
-        state = self.__dict__
+        state = self.__dict__.copy()
         state["data"] = self.data
         state["requires_grad"] = self.requires_grad
         return state

tests/test_linear4bit.py

@@ -186,19 +186,30 @@ def test_copy_param():
 def test_deepcopy_param():
     tensor = torch.tensor([1.0, 2.0, 3.0, 4.0])
     param = bnb.nn.Params4bit(data=tensor, requires_grad=False).cuda(0)
+    dict_keys_before = set(param.__dict__.keys())
     copy_param = copy.deepcopy(param)
+    dict_keys_after = set(param.__dict__.keys())
+    dict_keys_copy = set(copy_param.__dict__.keys())
+
     assert param.quant_state is not copy_param.quant_state
     assert param.data.data_ptr() != copy_param.data.data_ptr()
 
+    # there was a bug where deepcopy would modify the original object
+    assert dict_keys_before == dict_keys_after
+    assert dict_keys_before == dict_keys_copy
+
 
 def test_params4bit_real_serialization():
     original_tensor = torch.tensor([1.0, 2.0, 3.0, 4.0], dtype=torch.float32)
     original_param = bnb.nn.Params4bit(data=original_tensor, quant_type="fp4")
+    dict_keys_before = set(original_param.__dict__.keys())
 
     original_param.cuda(0)  # move to CUDA to trigger quantization
 
     serialized_param = pickle.dumps(original_param)
     deserialized_param = pickle.loads(serialized_param)
+    dict_keys_after = set(original_param.__dict__.keys())
+    dict_keys_deserialized = set(deserialized_param.__dict__.keys())
 
     assert torch.equal(original_param.data, deserialized_param.data)
     assert original_param.requires_grad == deserialized_param.requires_grad == False
@@ -206,3 +217,7 @@ def test_params4bit_real_serialization():
     assert original_param.blocksize == deserialized_param.blocksize
     assert original_param.compress_statistics == deserialized_param.compress_statistics
     assert original_param.quant_state == deserialized_param.quant_state
+
+    # there was a bug where deepcopy would modify the original object
+    assert dict_keys_before == dict_keys_after
+    assert dict_keys_before == dict_keys_deserialized