[BugFix] Add synchronize in CutlassW4A8LinearKernel to ensure data is ready for use. (#33078)

Co-authored-by: jinwuguo <jinwuguo@tencent.com>
Co-authored-by: Wentao Ye <44945378+yewentao256@users.noreply.github.com>

vllm/model_executor/layers/quantization/kernels/mixed_precision/cutlass.py

@@ -77,6 +77,7 @@ class CutlassW4A8LinearKernel(MPLinearKernel):
         def transform_w_q(x):
             assert isinstance(x, BasevLLMParameter)
             convert_packed_uint4b8_to_signed_int4_inplace(x.data)
+            torch.cuda.synchronize()
             permute_param_layout_(x, input_dim=0, output_dim=1, packed_dim=0)
             x.data = ops.cutlass_encode_and_reorder_int4b(x.data.t().contiguous().t())
             return x