Raise an error when loading a quantized checkpoint before quantization

bitsandbytes/nn/modules.py

@@ -248,6 +248,11 @@ class Linear8bitLt(nn.Linear):
         for key in unexpected_keys:
             input_name = key[len(prefix):]
             if input_name == "SCB":
+                if self.weight.SCB is None:
+                    # buffers not yet initialized, can't call them directly without
+                    raise RuntimeError("Loading a quantized checkpoint into non-quantized Linear8bitLt is "
+                                       "not supported. Please call module.cuda() before module.load_state_dict()")
+
                 input_param = state_dict[key]
                 self.weight.SCB.copy_(input_param)
                 unexpected_keys.remove(key)