fix warpctc

Author: JiayiFeng

paddle/fluid/operators/warpctc_op.h

@@ -186,8 +186,7 @@ class WarpCTCKernel : public framework::OpKernel<T> {
 
     // warpctc accesses labels in CPU memory
     Tensor warpctc_label;
-    TensorCopy(*label, platform::CPUPlace(), ctx.device_context(),
-               &warpctc_label);
+    TensorCopySync(*label, platform::CPUPlace(), &warpctc_label);
     const int* warpctc_label_data = warpctc_label.data<int>();
     // warpctc stores loss in CPU memory
     Tensor warpctc_loss;