data type of label is DType? it should be real_t?

there existed some problem in source file dmlc-core/src/data/row_block.h
```
struct RowBlockContainer {
/*! \brief array[size+1], row pointer to beginning of each rows */
std::vector offset;
/*! \brief array[size] label of each instance */
std::vector label;
/*! \brief array[size] weight of each instance */
std::vector weight;
/*! \brief array[size] session-id of each instance */
std::vector qid;
/*! \brief field index */
std::vector field;
/*! \brief feature index */
std::vector index;
```
the data type of label is real_t
@eric-haibin-lin