Fix src_idx out of range in beam_search_op

paddle/fluid/operators/beam_search_op.cc

@@ -87,7 +87,7 @@ void BeamSearch::PruneEndBeams(const framework::LoDTensor &pre_ids,
   auto *pre_ids_data = pre_ids.data<int64_t>();
   auto abs_lod = framework::ToAbsOffset(ids_->lod());
   auto &high_level = abs_lod[lod_level_];
-  for (size_t src_idx = 0; src_idx < high_level.size(); ++src_idx) {
+  for (size_t src_idx = 0; src_idx < high_level.size() - 1; ++src_idx) {
     size_t src_prefix_start = high_level[src_idx];
     size_t src_prefix_end = high_level[src_idx + 1];
     bool finish_flag = true;

python/paddle/fluid/tests/book/high-level-api/machine_translation/test_machine_translation.py

@@ -148,7 +148,11 @@ def decode(context, is_sparse):
         pd.array_write(selected_ids, array=ids_array, i=counter)
         pd.array_write(selected_scores, array=scores_array, i=counter)
 
-        pd.less_than(x=counter, y=array_len, cond=cond)
+        # update the break condition: up to the max length or all candidates of
+        # source sentences have ended.
+        length_cond = pd.less_than(x=counter, y=array_len)
+        finish_cond = pd.logical_not(pd.is_empty(x=selected_ids))
+        pd.logical_and(x=length_cond, y=finish_cond, out=cond)
 
     translation_ids, translation_scores = pd.beam_search_decode(
         ids=ids_array, scores=scores_array, beam_size=beam_size, end_id=10)

python/paddle/fluid/tests/book/test_machine_translation.py

@@ -147,7 +147,11 @@ def decoder_decode(context, is_sparse):
         pd.array_write(selected_ids, array=ids_array, i=counter)
         pd.array_write(selected_scores, array=scores_array, i=counter)
 
-        pd.less_than(x=counter, y=array_len, cond=cond)
+        # update the break condition: up to the max length or all candidates of
+        # source sentences have ended.
+        length_cond = pd.less_than(x=counter, y=array_len)
+        finish_cond = pd.logical_not(pd.is_empty(x=selected_ids))
+        pd.logical_and(x=length_cond, y=finish_cond, out=cond)
 
     translation_ids, translation_scores = pd.beam_search_decode(
         ids=ids_array, scores=scores_array, beam_size=beam_size, end_id=10)