CollectionDef.BytesList

 BytesList is used for collecting strings and serialized protobufs. For
 example:
 collection_def {
   key: "trainable_variables"
   value {
     bytes_list {
       value: "\n\017conv1/weights:0\022\024conv1/weights/Assign
              \032\024conv1/weights/read:0"
       value: "\n\016conv1/biases:0\022\023conv1/biases/Assign\032
              \023conv1/biases/read:0"
     }
   }
 }