set constant for loss

test=develop

paddle/fluid/operators/warpctc_cudnn_op.cu.cc

@@ -144,6 +144,8 @@ class CudnnCTCKernel : public framework::OpKernel<T> {
         CUDNN_CTC_LOSS_ALGO_DETERMINISTIC, cu_ctcloss_desc, &workspace_size));
 
     T* loss_data = loss->mutable_data<T>(loss_dims, ctx.GetPlace());
+    math::SetConstant<DeviceContext, T>()(
+        ctx.template device_context<DeviceContext>(), loss, static_cast<T>(0));
 
     auto temp_allocation =
         platform::DeviceTemporaryAllocator::Instance().Get(dev_ctx).Allocate(