[BUG] - Inconsistent default parameters between backtest and gridsearch

[thomktz]

Describe the bug
The backtest and gridsearch methods of forecasting models have inconsistent default values to their start (None v.s. 0.5) keyword argument. This makes the scores returned by gridsearch inconsistent with the scores manually computed through backtest with the same model parameters

Expected behavior
Set both to None or both to 0.5

**System

• darts version: 0.27.0

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