Add some comments

Author: bingyanghuang

paddle/fluid/operators/math/sequence_pooling.cc

@@ -109,24 +109,32 @@ class LastFirstSeqPoolFunctor {
   void operator()(const platform::CPUDeviceContext& context,
                   const framework::LoDTensor& input, framework::Tensor* output,
                   const std::string pooltype) {
+  //Create pointers to input and output data 
   auto* in_data = input.data<T>();
   auto* out_data = output->data<T>();
+
+  //Calculate length of each word
   int64_t word_len = input.numel() / input.dims()[0];
   auto lod = input.lod()[0];
-  auto dims = input.dims();
   if (pooltype == "LAST"){
       for (int i=0;  i < static_cast<int>(lod.size()) - 1; ++i ){
+            //Calculate the length of each sequence
             int64_t seq_len =  static_cast<int64_t>(lod[i + 1] - lod[i]);
+            //Point to the begin of next sequence
             in_data += seq_len* word_len;
-            std::memcpy(out_data,(in_data-word_len),word_len*sizeof(int));
+            //Copy the last words to output
+            std::memcpy(out_data,(in_data-word_len),word_len*sizeof(T));
             out_data += word_len;
 
        }
   }
   else if(pooltype == "FIRST"){
       for (int i=0;  i < static_cast<int>(lod.size()) - 1; ++i ){
+            //Calculate the length of each sequence 
             int64_t seq_len =  static_cast<int64_t>(lod[i + 1] - lod[i]);
-            std::memcpy(out_data,in_data,word_len*sizeof(int));
+            //Copy the first words of sequence to output
+            std::memcpy(out_data,in_data,word_len*sizeof(T));
+            //Point to the next sequence
             in_data += seq_len * word_len;
             out_data += word_len;