Helper to construct a sparse representation of the features.

def _construct_sparse_features(self, x):
        """ Helper to construct a sparse representation of the features. """
        I, J, K = x.shape
        new_array_height = (x != 0).sum(axis=2).max()
        index_array = -np.ones((I, J, new_array_height), dtype='int64')
        value_array = -np.ones((I, J, new_array_height), dtype='float64')
        populate_sparse_features(x, index_array, value_array, I, J, K)
        return index_array, value_array