stl_arima serialization bug

[nicl-nno]

Pipeline with stl_arima can not be serialized correctly with pipeline.save()

[kasyanovse]

Saving works now.

Code for testing:

import numpy as np

from fedot.core.data.data import InputData
from fedot.core.pipelines.node import PipelineNode
from fedot.core.pipelines.pipeline import Pipeline
from fedot.core.repository.dataset_types import DataTypesEnum
from fedot.core.repository.tasks import Task, TaskTypesEnum, TsForecastingParams

features = np.random.rand(1000)

data = InputData(idx=np.arange(len(features)),
                 features=features,
                 target=features,
                 data_type=DataTypesEnum.ts,
                 task=Task(TaskTypesEnum.ts_forecasting, TsForecastingParams(10)))

pipeline = Pipeline(PipelineNode('stl_arima'))
pipeline.fit(data)
pipeline.save(r'D:\buffer')