Support split ratio for ts forecasting task

[Dreamlone]

The split factor for the time series is now ignored. There is a need to implement time series in-sample forecasting to use this info during validation

_sample_split_ratio_for_tasks = {
    TaskTypesEnum.classification: 0.8,
    TaskTypesEnum.regression: 0.8,
    TaskTypesEnum.ts_forecasting: 0.5
}