* Split up column matrix initialization. This PR splits the column matrix initialization into 2 steps, the first one initializes the storage while the second one does the transpose. By doing so, we can reuse the code for Quantile DMatrix.
* Split up column matrix initialization. This PR splits the column matrix initialization into 2 steps, the first one initializes the storage while the second one does the transpose. By doing so, we can reuse the code for Quantile DMatrix.