Make an n-fold list of CVPack from random indices.

def mknfold(dall, nfold, param, seed, evals=(), fpreproc=None):
    """
    Make an n-fold list of CVPack from random indices.
    """
    evals = list(evals)
    np.random.seed(seed)
    randidx = np.random.permutation(dall.num_row())
    kstep = len(randidx) / nfold
    idset = [randidx[(i * kstep): min(len(randidx), (i + 1) * kstep)] for i in range(nfold)]
    ret = []
    for k in range(nfold):
        dtrain = dall.slice(np.concatenate([idset[i] for i in range(nfold) if k != i]))
        dtest = dall.slice(idset[k])
        # run preprocessing on the data set if needed
        if fpreproc is not None:
            dtrain, dtest, tparam = fpreproc(dtrain, dtest, param.copy())
        else:
            tparam = param
        plst = list(tparam.items()) + [('eval_metric', itm) for itm in evals]
        ret.append(CVPack(dtrain, dtest, plst))
    return ret