Exp RMSE between `pred` and `targ`.

def exp_rmspe(pred:Tensor, targ:Tensor)->Rank0Tensor:
    "Exp RMSE between `pred` and `targ`."
    pred,targ = flatten_check(pred,targ)
    pred, targ = torch.exp(pred), torch.exp(targ)
    pct_var = (targ - pred)/targ
    return torch.sqrt((pct_var**2).mean())