Merge pull request #13012 from tensor-tang/refine/seq2batch

refine seq2batch

paddle/fluid/operators/math/sequence2batch.cc

@@ -38,13 +38,14 @@ class CopyMatrixRowsFunctor<platform::CPUDeviceContext, T> {
     auto width = dst_dims[1];
     auto* src_data = src.data<T>();
     auto* dst_data = dst->data<T>();
-    for (int i = 0; i < height; ++i) {
-      if (is_src_index) {
-        memcpy(dst_data + i * width, src_data + index[i] * width,
-               width * sizeof(T));
-      } else {
-        memcpy(dst_data + index[i] * width, src_data + i * width,
-               width * sizeof(T));
+    const int sz = width * sizeof(T);
+    if (is_src_index) {
+      for (int i = 0; i < height; ++i) {
+        memcpy(dst_data + i * width, src_data + index[i] * width, sz);
+      }
+    } else {
+      for (int i = 0; i < height; ++i) {
+        memcpy(dst_data + index[i] * width, src_data + i * width, sz);
       }
     }
   }