[AutoML] Independent control of the backtest window size and forecast horizon

[ncooder]

Description

Please add a new parameter for AutoMLforecast that will disconnect the forecast horizon and horizon for backtest windows.

Use case

No response

[jmoralez]

Hey. Do you mean the step_size? i.e. by how many timestamps the train end advances in each window? #419 added it to the objective function so it'd just be a matter of adding it to the AutoMLforecast class

[ncooder]

@jmoralez Yes, I was referring exactly to this. Thank you!