fix task.py with no dest_sample_rate task; fix bug in train and infer

mengzhe.cmz

2023-04-12 1ad439f96b8cdbca98ee95b886d692dfa20e85ce

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#include "yaml-cpp/null.h"
 
namespace YAML {
_Null Null;
 
bool IsNullString(const std::string& str) {
  return str.empty() || str == "~" || str == "null" || str == "Null" ||
         str == "NULL";
}
}