Fix copying empty tensor in beam_search_decode_op

paddle/fluid/operators/beam_search_decode_op.cc

@@ -42,9 +42,11 @@ struct BeamSearchDecodeFunctor {
       // Copy all tensors in the input tensor array
       for (auto& step_id : step_ids_origin_) {
         framework::LoDTensor out;
-        dev_ctx->Wait();
-        framework::TensorCopy(step_id, platform::CPUPlace(), *dev_ctx, &out);
-        dev_ctx->Wait();
+        if (step_id.numel() > 0) {
+          dev_ctx->Wait();
+          framework::TensorCopy(step_id, platform::CPUPlace(), *dev_ctx, &out);
+          dev_ctx->Wait();
+        }
 
         out.set_lod(step_id.lod());
         step_ids_.push_back(out);
@@ -58,9 +60,12 @@ struct BeamSearchDecodeFunctor {
       // Copy all tensors in the input tensor array
       for (auto& step_score : step_scores_origin_) {
         framework::LoDTensor out;
-        dev_ctx->Wait();
-        framework::TensorCopy(step_score, platform::CPUPlace(), *dev_ctx, &out);
-        dev_ctx->Wait();
+        if (step_score.numel() > 0) {
+          dev_ctx->Wait();
+          framework::TensorCopy(step_score, platform::CPUPlace(), *dev_ctx,
+                                &out);
+          dev_ctx->Wait();
+        }
 
         out.set_lod(step_score.lod());
         step_scores_.push_back(out);

paddle/fluid/operators/beam_search_decode_op.h

@@ -151,8 +151,7 @@ void BeamSearchDecoder<T>::Backtrace(const LoDTensorArray& step_ids,
   const size_t src_num = step_ids.at(0).lod().at(kSourceLevel).size() - 1;
   std::vector<SentenceVector<T>> sentence_vector_list(
       src_num, SentenceVector<T>(beam_size_));
-  std::vector<std::vector<size_t>> prefix_idx_vector_list(
-      src_num, std::vector<size_t>());
+  std::vector<std::vector<size_t>> prefix_idx_vector_list(src_num);
   for (int step_id = step_num - 1; step_id >= 0; --step_id) {
     auto& cur_ids = step_ids.at(step_id);
     auto& cur_scores = step_scores.at(step_id);