fix _functions

Author: Lzy17

bitsandbytes/autograd/_functions.py

@@ -464,7 +464,7 @@ def backward(ctx, grad_output):
             grad_output = grad_output.reshape(-1, grad_output.shape[-1]).contiguous()
 
         Cgrad, Cgradt, SCgrad, SCgradt, coo_tensor = None, None, None, None, None
-        if req_gradB or (req_gradA and state.CBt):
+        if req_gradB or (req_gradA and state.CBt is not None):
             Cgrad, Cgradt, SCgrad, SCgradt, coo_tensor = F.double_quant(grad_output.to(torch.float16))
         if req_gradB:
             CxAt, SAt = F.transform(CAt, formatB, transpose=True)