Support more flexible prediction range or improve documentation about prediction range.

[Eve-Chen]

Is your feature request related to a problem? Please describe. If forecast range starts after train end date, the forecaster interface assumes the forecast range starts right after train end date (this line). Can we remove that hard assumption and allow a gap between train end date and forecast start date? i.e. prediction range = [train end date + gap, train end date + gap + n_forecast_steps].

Describe the solution you'd like Support generic scenario where prediction range = [train start date + x, train start date + x + n_forecast_steps]. This would cover the current supported use cases, and the case where there is a gap between prediction range and train range.

Describe alternatives you've considered Explicitly state the assumption in API documentation, and add warning about the gap (if exists) and the current handling about the gap.

Additional context Add any other context or screenshots about the feature request here.