Feat/wrapper model gridsearch

[madtoinou]

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• Mentioned all issues that this PR fixes or addresses.
• Summarized the updates of this PR under Summary.
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Fixes #2104.

Summary

Copied from #2133.

When the model key is found in the parameters dictionary passed to the gridsearch classmethod (meaning we are in the context of a model that wraps another), the class method expects either a list of wrapped model instances , or it expects a dictionary with a special key called model_class whose value is the class of the model to be wrapped. The other keys in the dictionary are the parameters that will be used to construct the grid dedicated to the wrapped model. Example

from sklearn.ensemble import RandomForestRegressor

from darts.models import RegressionModel
from darts.utils import timeseries_generation as tg

parameters = {
    "model": {
        "model_class": RandomForestRegressor,
        "min_samples_split": [2,3],
        "min_samples_leaf": [1,2],
    },
    "lags": [1,2,3],
}
series = tg.sine_timeseries(length=100)

RegressionModel.gridsearch(
    parameters=parameters, series=series, forecast_horizon=1
)

[andresliszt]

Seems that you added a validator on input parameters, the wrapped model dictionary is not allowed anymore. Here, I think might be loosen with an except for the model key

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.10%. Comparing base (bb24999) to head (c9f97f4).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2594      +/-   ##
==========================================
- Coverage   94.14%   94.10%   -0.05%     
==========================================
  Files         139      139              
  Lines       14884    14904      +20     
==========================================
+ Hits        14013    14025      +12     
- Misses        871      879       +8     

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