Complete shrink_rnn_memory_op comments (PaddlePaddle#5935)

* Complete shrink_rnn_memory_op comments * Update

Author: JiayiFeng

paddle/operators/shrink_rnn_memory_op.cc

@@ -57,11 +57,21 @@ class ShrinkRNNMemoryOpProtoMaker : public framework::OpProtoAndCheckerMaker {
  ShrinkRNNMemoryOpProtoMaker(framework::OpProto *proto,
  framework::OpAttrChecker *op_checker)
  : OpProtoAndCheckerMaker(proto, op_checker) {
- AddInput("X", "");
- AddInput("RankTable", "");
- AddInput("I", "");
- AddOutput("Out", "");
- AddComment("");
+ AddInput("X", "(LoDTensor) The RNN step memory to be shrinked.");
+ AddInput("RankTable", "(LoDRankTable) The lod_rank_table of dynamic RNN.");
+ AddInput("I",
+ "(LoDTensor) The step index. The RNN step memory 'X' will be "
+ "shrinked to match the size of the input of the index'th step.");
+ AddOutput("Out", "(LoDTensor) The shrinked RNN step memory.");
+ AddComment(
+ R"DOC(
+ In dynamic RNN, we are able to handle sequences of different lengths. 
+ Because of the multiple lengths, the size of each step input can be 
+ different, which may lead to a mismatching between the input of
+ the current step and the memory generated by the previous one. This 
+ operator shrinks memory according to the size of the next step input, 
+ to make sure that they can match each other.
+ )DOC");
  }
 };