fix bug in merge_ids (#15503)

* fix mistakes in merge_ids, test=develop

paddle/fluid/operators/distributed_ops/merge_ids_op.h

@@ -43,9 +43,9 @@ class MergeIdsOpKernel : public framework::OpKernel<T> {
     PADDLE_ENFORCE_EQ(ids.size(), outs.size(),
                       "the number of Ids and Out should be the same");
 
-    size_t row_ids_size = 0;
-    int row_size = 0;
-    int embedding_size = 0;
+    int64_t row_ids_size = 0;
+    int64_t row_size = 0;
+    int64_t embedding_size = 0;
 
     for (size_t i = 0; i < x_tensors.size(); ++i) {
       const auto *x_tensor = x_tensors[i];
@@ -69,7 +69,7 @@ class MergeIdsOpKernel : public framework::OpKernel<T> {
     for (size_t i = 0; i < x_tensors.size(); ++i) {
       const auto *row_id = row_ids[i];
 
-      for (int j = 0; j < row_id->numel(); ++j) {
+      for (auto j = 0; j < row_id->numel(); ++j) {
         int64_t key = row_id->data<int64_t>()[j];
         std::tuple<int64_t, int64_t> val = std::make_tuple(i, j);
         selected_rows_idx_map.insert(std::make_pair(key, val));
@@ -84,13 +84,13 @@ class MergeIdsOpKernel : public framework::OpKernel<T> {
 
       out->set_lod(out_ids->lod());
 
-      int nums = static_cast<int>(out_ids->dims()[0]);
+      auto nums = out_ids->dims()[0];
       auto *out_data = out->mutable_data<T>(
           framework::make_ddim({nums, embedding_size}), place);
-      for (int j = 0; j < nums; ++j) {
-        int id = out_ids->data<int64_t>()[j];
-        auto row_tuple = selected_rows_idx_map[id];
-        int64_t row_idx = std::get<1>(row_tuple);
+      for (auto j = 0; j < nums; ++j) {
+        auto id = out_ids->data<int64_t>()[j];
+        auto row_tuple = selected_rows_idx_map.at(id);
+        auto row_idx = std::get<1>(row_tuple);
         const auto *x_tensor = x_tensors[std::get<0>(row_tuple)];
 
         memcpy(out_data + embedding_size * j,