Add comment of GetSubLoDAndAbsoluteOffset (#6771)

paddle/framework/lod_tensor.h

@@ -184,6 +184,18 @@ LoDTensor LodExpand(const LoDTensor& source, const LoD& lod, size_t level,
   return tensor;
 }
 
+// Get the absolute offset of a lod[start_level][start_idx:end_idx] and
+// relative length of details for every levels(i.e., [start_level: ]).
+//
+// For example,
+//   lod = [[0, 3, 4, 8], [0, 9, 10, 11, 13, 17, 19, 22, 24]]
+//   start_level = 0
+//   start_idx = 1
+//   end_idx = 3
+//
+// Returns:
+//  LoD = [[1, 4], [2, 4, 2, 3, 2]]
+//  pair<size_t, size_t> = {11, 24}
 std::pair<LoD, std::pair<size_t, size_t>> GetSubLoDAndAbsoluteOffset(
     const LoD& lod, size_t start_idx, size_t end_idx, size_t start_level);