[bitsandbytes] Simplify bnb int8 dequant (#10401)

* fix dequantization for latest bnb.

* smol fixes.

* fix type annotation

* update peft link

* updates

Author: sayakpaul

src/diffusers/quantizers/bitsandbytes/utils.py

@@ -153,8 +153,8 @@ def replace_with_bnb_linear(model, modules_to_not_convert=None, current_key_name
     return model
 
 
-# Copied from PEFT: https://github.com/huggingface/peft/blob/47b3712898539569c02ec5b3ed4a6c36811331a1/src/peft/utils/integrations.py#L41
-def dequantize_bnb_weight(weight: "torch.nn.Parameter", state=None):
+# Adapted from PEFT: https://github.com/huggingface/peft/blob/6d458b300fc2ed82e19f796b53af4c97d03ea604/src/peft/utils/integrations.py#L81
+def dequantize_bnb_weight(weight: "torch.nn.Parameter", state=None, dtype: "torch.dtype" = None):
     """
     Helper function to dequantize 4bit or 8bit bnb weights.
 
@@ -177,13 +177,16 @@ def dequantize_bnb_weight(weight: "torch.nn.Parameter", state=None):
     if state.SCB is None:
         state.SCB = weight.SCB
 
-    im = torch.eye(weight.data.shape[-1]).contiguous().half().to(weight.device)
-    im, imt, SCim, SCimt, coo_tensorim = bnb.functional.double_quant(im)
-    im, Sim = bnb.functional.transform(im, "col32")
-    if state.CxB is None:
-        state.CxB, state.SB = bnb.functional.transform(weight.data, to_order=state.formatB)
-    out32, Sout32 = bnb.functional.igemmlt(im, state.CxB, Sim, state.SB)
-    return bnb.functional.mm_dequant(out32, Sout32, SCim, state.SCB, bias=None).t()
+    if hasattr(bnb.functional, "int8_vectorwise_dequant"):
+        # Use bitsandbytes API if available (requires v0.45.0+)
+        dequantized = bnb.functional.int8_vectorwise_dequant(weight.data, state.SCB)
+    else:
+        # Multiply by (scale/127) to dequantize.
+        dequantized = weight.data * state.SCB.view(-1, 1) * 7.874015718698502e-3
+
+    if dtype:
+        dequantized = dequantized.to(dtype)
+    return dequantized
 
 
 def _create_accelerate_new_hook(old_hook):
@@ -205,6 +208,7 @@ def _create_accelerate_new_hook(old_hook):
 
 def _dequantize_and_replace(
     model,
+    dtype,
     modules_to_not_convert=None,
     current_key_name=None,
     quantization_config=None,
@@ -244,7 +248,7 @@ def _dequantize_and_replace(
                 else:
                     state = None
 
-                new_module.weight = torch.nn.Parameter(dequantize_bnb_weight(module.weight, state))
+                new_module.weight = torch.nn.Parameter(dequantize_bnb_weight(module.weight, state, dtype))
 
                 if bias is not None:
                     new_module.bias = bias
@@ -263,9 +267,10 @@ def _dequantize_and_replace(
         if len(list(module.children())) > 0:
             _, has_been_replaced = _dequantize_and_replace(
                 module,
-                modules_to_not_convert,
-                current_key_name,
-                quantization_config,
+                dtype=dtype,
+                modules_to_not_convert=modules_to_not_convert,
+                current_key_name=current_key_name,
+                quantization_config=quantization_config,
                 has_been_replaced=has_been_replaced,
             )
         # Remove the last key for recursion
@@ -280,6 +285,7 @@ def dequantize_and_replace(
 ):
     model, has_been_replaced = _dequantize_and_replace(
         model,
+        dtype=model.dtype,
         modules_to_not_convert=modules_to_not_convert,
         quantization_config=quantization_config,
     )