Change default gridsearch kwarg value (#2243)

* Change default kwarg

* Update CHANGELOG.md

---------

Co-authored-by: Dennis Bader <dennis.bader@gmx.ch>

CHANGELOG.md

@@ -21,6 +21,8 @@ but cannot always guarantee backwards compatibility. Changes that may **break co
   - Added support for additional lr scheduler configuration parameters for more control ("interval", "frequency", "monitor", "strict", "name"). [#2218](https://github.com/unit8co/darts/pull/2218) by [Dennis Bader](https://github.com/dennisbader).
 - Improvements to `WindowTransformer` and `window_transform`:
   - Added argument `keep_names` to indicate whether the original component names should be kept. [#2207](https://github.com/unit8co/darts/pull/2207)by [Antoine Madrona](https://github.com/madtoinou).
+- Other improvements:
+  - 🔴 Changed the default `start` value in `ForecastingModel.gridsearch()` from `0.5` to `None`, to make it consistent with `historical_forecasts` and other methods. [#2243](https://github.com/unit8co/darts/pull/2243) by [Thomas Kientz](https://github.com/thomktz).
 
 **Fixed**
 - Fixed a bug when calling `window_transform` on a `TimeSeries` with a hierarchy. The hierarchy is now only preseved for single transformations applied to all components, or removed otherwise. [#2207](https://github.com/unit8co/darts/pull/2207)by [Antoine Madrona](https://github.com/madtoinou).

darts/models/forecasting/forecasting_model.py

@@ -1344,7 +1344,7 @@ def gridsearch(
         future_covariates: Optional[TimeSeries] = None,
         forecast_horizon: Optional[int] = None,
         stride: int = 1,
-        start: Union[pd.Timestamp, float, int] = 0.5,
+        start: Optional[Union[pd.Timestamp, float, int]] = None,
         start_format: Literal["position", "value"] = "value",
         last_points_only: bool = False,
         show_warnings: bool = True,