fix parse content in CreatePreLoadReaders (#20258) (#20570)

Fix parse content in CreatePreLoadReaders. Before this fix, if you use dataset.set_parse_content and dataset.preload, parse content didn't work.

paddle/fluid/framework/data_set.cc

@@ -561,6 +561,7 @@ void DatasetImpl<T>::CreatePreLoadReaders() {
     preload_readers_[i]->SetFileListIndex(&file_idx_);
     preload_readers_[i]->SetFileList(filelist_);
     preload_readers_[i]->SetParseInsId(parse_ins_id_);
+    preload_readers_[i]->SetParseContent(parse_content_);
     preload_readers_[i]->SetInputChannel(input_channel_.get());
     preload_readers_[i]->SetOutputChannel(nullptr);
     preload_readers_[i]->SetConsumeChannel(nullptr);