[BUG] Gridsearch: Error metrics in split window mode - misleading order of method parameters.

[ClaraGrthns]

Describe the bug
In split window mode, the order of parameters is misleading. Normally, metrics like mape provided by darts get the actual series first and then the predicted series as input: darts.metrics.metrics.mape(actual_series, pred_series,...).

In the split-window mode of the gridsearch method, this is reversed, leading to incorrect results when comparing the different model configurations:

In darts.models.forecasting.forecasting_model.py (line 784):

@classmethod
    def gridsearch(
 ...

      else:  # split mode
                      model._fit_wrapper(series, past_covariates, future_covariates)
                      pred = model._predict_wrapper(
                          len(val_series),
                          series,
                          past_covariates,
                          future_covariates,
                          num_samples=1,
                      )
                      error = metric(pred, val_series)
      
                  return float(error)
...

Expected behavior
error = metric(val_series, pred).

System:

• Python version: [e.g. 3.9.11]
• darts version [e.g. 0.19.0]

[hrzn]

Good catch! Do you want to open a PR with the fix you propose ?