text: Title: [Question] Does backtest method automatically apply inverse transformation for mappings?
  Body: I'm currently studying Darts and it is clear to me how the backtest method works when I pass a mapped series to it as an argument.
  
  For example, if I write
  
  `best_model.backtest(series[0].map(np.log1p), forecast_horizon=1, metric=mae)`
  
  where `best_model` is a Theta model object and mae was imported from Darts, will it infer the inverse transformation (in this case, np.expm1) and apply it to the forecasted data in order to calculate the metrics or not?
  
  Thank you in advance.