[BUG] ForecastingModel.backtest: Can bypass sanity checks

[LeoTafti]

Describe the bug
Invalid optional arguments to ForecastingModel.backtest can bypass sanity checks when provided as positional arguments

To Reproduce

NaiveDrift().backtest(linear_series, None, start=0.7, forecast_horizon=-1)
NaiveDrift().backtest(linear_series, None, 0.7, -1)

Expected behavior
The first line above is caught by the sanity checks and raise ValueError: 'The provided forecasting horizon must be a positive integer.' as expected

For the second line however the negative forecast_horizon param clears the sanity checks and produces an error further down the line:

NaiveDrift().backtest(linear_series, None, 0.7, -1)
  File ".../darts/utils/utils.py", line 120, in sanitized_method
    return method_to_sanitize(self, *args, **kwargs)
  File ".../darts/models/forecasting_model.py", line 217, in backtest
    pred = self.predict(forecast_horizon, **predict_kwargs)
  File ".../darts/models/baselines.py", line 98, in predict
    forecast = np.linspace(last, last_value, num=n)
  File "<__array_function__ internals>", line 5, in linspace
  File ".../lib/python3.8/site-packages/numpy/core/function_base.py", line 115, in linspace
    raise ValueError("Number of samples, %s, must be non-negative." % num)
ValueError: Number of samples, -1, must be non-negative.

System (please complete the following information):

• Python version: 3.8.5
• darts version: develop branch, commit #2977f4f

[pennfranc]

Good catch! That's definitely a drawback of the current sanity checks implementation. It mostly relies on arguments being passed as keywords right now.. Do you think this could be a possible solution: https://stackoverflow.com/a/831164?