I think I understand how MASE works when I have a single time series. But what if I have several, for which I want to obtain an overall accuracy measure?
It's straightforward to compute an aggregate statistic for other measures (MAPE, MAE, RMSE, etc), since there are only 2 vectors of numbers to consider: the predicted values and the actuals. But with MASE, you also need to take into consideration how to compute the scaling factor, which requires a training set.
Code that uses Rob Hyndman's Tidyverts framework would also be great to see, but not essential.