[bitsandbytes] improve dtype mismatch handling for bnb + lora. (#11270)

* improve dtype mismatch handling for bnb + lora.

* add a test

* fix and updates

* update

src/diffusers/quantizers/bitsandbytes/utils.py

@@ -171,9 +171,11 @@ def dequantize_bnb_weight(weight: "torch.nn.Parameter", state=None, dtype: "torc
 
     if cls_name == "Params4bit":
         output_tensor = bnb.functional.dequantize_4bit(weight.data, weight.quant_state)
-        logger.warning_once(
-            f"The model is going to be dequantized in {output_tensor.dtype} - if you want to upcast it to another dtype, make sure to pass the desired dtype when quantizing the model through `bnb_4bit_quant_type` argument of `BitsAndBytesConfig`"
-        )
+        msg = f"The model is going to be dequantized in {output_tensor.dtype} - if you want to upcast it to another dtype, make sure to pass the desired dtype when quantizing the model through `bnb_4bit_quant_type` argument of `BitsAndBytesConfig`"
+        if dtype:
+            msg = f"The model is going to be first dequantized in {output_tensor.dtype} and type-casted to {dtype}"
+            output_tensor = output_tensor.to(dtype)
+        logger.warning_once(msg)
         return output_tensor
 
     if state.SCB is None: