code: public static INDArray toMatrix(List<List<Writable>> records) {
          List<INDArray> toStack = new ArrayList<>();
          for(List<Writable> l : records){
              toStack.add(toArray(l));
          }
  
          return Nd4j.vstack(toStack);
      }
docs: Convert a set of records in to a matrix
  @param records the records ot convert
  @return the matrix for the records